System and method for temperature sensing using thermopile integrated with rigid printed circuit board

ABSTRACT

Robust estimation of temperatures inside and outside a device can be achieved using one or more absolute temperature sensors optionally in conjunction with thermopile heat flux sensors. Thermopile temperature sensing systems can measure a temperature gradient across two locations within the device, to estimate absolute temperature at locations that are impractical to measure using absolute temperature sensors. Using heat flux models associated with the device, the thermopile temperature sensing system can be used to estimate temperature associated with objects that contact an outer surface of the device, such as a user&#39;s skin temperature. Additionally, the thermopile temperature sensing system can be used to estimate ambient air temperature. Within a device, temperature measurements from the thermopile temperature sensors can be used to compensate sensor measurements, such as when the accuracy or reliability of a sensor varies with temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.63/261,659, filed Sep. 24, 2021, U.S. Provisional Application No.63/261,660, filed Sep. 24, 2021, U.S. Provisional Application No.63/261,661, filed Sep. 24, 2021, U.S. Provisional Application No.63/261,663, filed Sep. 25, 2021, and U.S. Provisional Application No.63/371,820, filed Aug. 18, 2022, the contents of which are incorporatedherein by reference in their entireties for all purposes.

FIELD OF THE DISCLOSURE

This relates generally to temperature sensing systems and methods, andmore particularly, to temperature sensing systems and methods using athermopile and/or multiple temperature sensors.

BACKGROUND OF THE DISCLOSURE

Many types of electronic devices include a temperature sensor formeasuring a temperature. The temperature sensor can measure atemperature at a location within the electronic device, and provide asignal corresponding to its temperature measurement to a processor onthe electronic device.

SUMMARY OF THE DISCLOSURE

In some examples, systems and methods for temperature sensing using athermopile (a collection of series-connected thermocouples) aredisclosed. An electronic device can leverage measurements from multiplesensors—including at least one absolute temperature sensor and at leastone other absolute or gradient sensor such as at least one thermopiletemperature sensor—within the device, to estimate temperature inside oroutside the device. In some examples, an electronic device estimates thetemperature of the surrounding air (e.g., ambient air temperature) usingmeasurements from sensors within the device. In other examples, anelectronic device estimates the temperature of objects (e.g., skin orbody temperature) contacting one or more of its surfaces (e.g., a rearsurface of the device, a front surface, etc.). In some examples, one ormore absolute temperature sensors within the electronic device can bedisposed on or otherwise integrated with one or more printed circuitboard (PCB) (e.g., a logic board, a system-in-package, a display, etc.).In some examples, a thermopile temperature can be configured to measurea temperature differential inside the device. In some examples, athermopile can be embedded within or otherwise integrated with a rigidcircuit board (e.g., a rigid PCB). In some examples, a thermopile can beembedded, or otherwise integrated, within a flexible printed circuit(FPC, also referred to herein as a flexible circuit or flex circuit). Insome examples, the thermopile can be embedded or otherwise integratedwithin rigid PCBs and FPCs that are used within an electronic device forother purposes to reduce the additional space required for temperatureand/or heat flux sensing by the device. A temperature differentialmeasurement (temperature gradient measurement) of the thermopiletemperature sensor can be used for inferring heat flux (e.g., throughthe electronic device) and/or to estimate temperatures outside thedevice (e.g., air temperature around the device, surface temperature ofobjects contacting the device housing, body temperature of a userwearing the device, etc.).

In some examples, temperature sensing systems of electronic devices andmethods that utilize multiple temperature sensors are disclosed. Anelectronic device can leverage measurements from a plurality oftemperature sensors, including, for example, a first absolutetemperature sensor at a first location in the device and a secondabsolute temperature sensor at a second location in the device, toestimate temperature inside or outside the device (e.g., temperature ofthe surrounding air (e.g., ambient air temperature) or the temperatureof objects (e.g., skin or body temperature) contacting one or more ofits surfaces (e.g., a rear surface of the device, a front surface,etc.)) using measurements from the temperature sensors of the device,optionally in addition to thermal resistance values at various locationsinside and/or outside the device. In some examples, the first absolutetemperature sensor is disposed on or otherwise integrated with a firstprinted circuit board (PCB) (e.g., a logic board, a main logic board, asystem-in-package component, a display, etc.) and the second absolutetemperature sensor is disposed on or otherwise integrated with a secondPCB different from the first PCB. In some examples, estimatingtemperatures outside of the electronic device is a function of a firsttemperature measurement from the first absolute temperature sensor, asecond temperature measurement from the second absolute temperaturemeasurement, and one or more thermal resistance values corresponding tovarious locations or regions inside and/or outside the device. In someexamples, an electronic device leverages measurements from a pluralityof temperature sensors, including, for example, at least two temperaturesensors (e.g., 2, 3, or 4 temperature sensors), to measure and/orestimate ambient temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1G illustrate exemplary systems with heat flux and/ortemperature sensors configured for measuring or estimating temperatureswithin and/or outside the system according to some examples of thedisclosure.

FIG. 2 illustrates a block diagram of an exemplary electronic devicethat includes a temperature sensing system according to some examples ofthe disclosure.

FIG. 3 illustrates a schematic view for an exemplary configuration of athermopile device and a thermopile measurement circuit according to someexamples of the disclosure.

FIG. 4A illustrates a cross-sectional side view of an exemplaryelectronic device including one or more printed circuit boards andtemperature sensing circuitry according to some examples of thedisclosure.

FIG. 4B illustrates a schematic view for an exemplary configuration thatincludes multiple thermopile devices according to some examples of thedisclosure.

FIGS. 5A and 5B illustrate simplified schematic views of heat fluxmodels for an electronic device relative to a user's body according tosome examples of the disclosure.

FIG. 6A illustrates a cross-sectional side view of an exemplaryelectronic device with temperature sensing circuitry and/or heat fluxsensing circuitry integrated with a printed circuit board according tosome examples of the disclosure.

FIG. 6B illustrates an exploded view of a portion of an exemplaryelectronic device including one or more printed circuit boards and anintegrated thermopile according to some examples of the disclosure.

FIG. 7A illustrates a cross-sectional side view of an exemplarytwo-layer rigid printed circuit board with an integrated thermopile, andoptional circuit components mounted on top and/or bottom surfaces of theprinted circuit board according to some examples of the disclosure.

FIG. 7B illustrates a plan view of the exemplary rigid printed circuitboard with an integrated thermopile according to some examples of thedisclosure.

FIG. 8 illustrates a cross-sectional side view of an exemplaryfour-layer rigid printed circuit board with an integrated thermopilespanning four layers according to some examples of the disclosure.

FIG. 9 illustrates a cross-sectional side view of an exemplaryfour-layer rigid printed circuit board with an integrated thermopilespanning three layers according to some examples of the disclosure.

FIG. 10 illustrates a cross-sectional side view of an exemplaryfour-layer rigid printed circuit board with an integrated thermopilespanning two layers according to some examples of the disclosure.

FIG. 11 illustrates a cross-sectional side view of an exemplaryelectronic device with temperature sensing circuitry and/or heat sensingcircuitry integrated with a flexible circuit according to some examplesof the disclosure.

FIG. 12A illustrates a cross-sectional side view of an exemplaryflexible circuit with an inner layer used for signal propagation andouter ground layers that protect the inner layer according to someexamples of the disclosure.

FIG. 12B illustrates a cross-sectional side view of the exemplaryflexible circuit of FIG. 12A with an integrated thermopile according tosome examples of the disclosure.

FIG. 13A illustrates a cross-sectional side view of an exemplaryflexible circuit with a first segment using an inner layer for datasignal propagation and a second segment using an outer layer (or outerlayers) for power signal propagation according to some examples of thedisclosure.

FIG. 13B illustrates a cross-sectional side view of the exemplaryflexible circuit of FIG. 13A with an integrated thermopile according tosome examples of the disclosure.

FIG. 14 illustrates an example process of estimating a temperatureinside and/or outside a device according to some examples of thedisclosure.

FIG. 15 illustrates an example process of operating a device fortemperature sensing operations according to some examples of thedisclosure.

FIG. 16 illustrates another example process of operating a device fortemperature sensing operations according to some examples of thedisclosure.

FIG. 17 illustrates another example process of operating a device fortemperature sensing operations according to some examples of thedisclosure.

FIG. 18 illustrates a cross-sectional side view of an exemplaryelectronic device including one or more printed circuit boards andtemperature sensing circuitry according to some examples of thedisclosure.

FIG. 19 illustrates simplified schematic view of a heat flux model foran electronic device relative to a user's body according to someexamples of the disclosure.

DETAILED DESCRIPTION

In the following description of examples, reference is made to theaccompanying drawings which form a part hereof, and in which it is shownby way of illustration specific examples that are optionally practiced.It is to be understood that other examples are optionally used andstructural changes are optionally made without departing from the scopeof the disclosed examples.

This relates to systems and methods for temperature sensing using athermopile (a collection of series-connected thermocouples). Anelectronic device can leverage measurements from multiplesensors—including at least one absolute temperature sensor and at leastone other absolute or gradient sensor such as at least one thermopiletemperature sensor—within the device, to estimate temperature inside oroutside the device. In some examples, an electronic device estimates thetemperature of the surrounding air (e.g., ambient air temperature) usingmeasurements from sensors within the device. In other examples, anelectronic device estimates the temperature of objects (e.g., skin orbody temperature) contacting one or more of its surfaces (e.g., a rearsurface of the device, a front surface, etc.). In some examples, one ormore absolute temperature sensors within the electronic device can bedisposed on or otherwise integrated with one or more printed circuitboard (PCB) (e.g., a logic board, a system-in-package, a display, etc.).In some examples, a thermopile temperature sensor can be configured tomeasure a temperature differential inside the device. In some examples,a thermopile can be embedded within or otherwise integrated with a rigidcircuit board (e.g., a rigid PCB). In some examples, a thermopile can beembedded, or otherwise integrated, within a flexible printed circuit(FPC, also referred to herein as a flexible circuit or flex circuit). Insome examples, the thermopile can be embedded or otherwise integratedwithin rigid PCBs and FPCs that are used within an electronic device forother purposes to reduce the additional space required for temperatureand/or heat flux sensing by the device. A temperature differentialmeasurement (temperature gradient measurement) of the thermopiletemperature sensor can be used for inferring heat flux (e.g., throughthe electronic device) and/or to estimate temperatures outside thedevice (e.g., air temperature around the device, surface temperature ofobjects contacting the device housing, body temperature of a userwearing the device, etc.).

This also relates to temperature sensing systems of electronic devicesand methods that utilize multiple temperature sensors. An electronicdevice can leverage measurements from a plurality of temperaturesensors, including, for example, a first absolute temperature sensor ata first location in the device and a second absolute temperature sensorat a second location in the device, to estimate temperature inside oroutside the device (e.g., temperature of the surrounding air (e.g.,ambient air temperature) or the temperature of objects (e.g., skin orbody temperature) contacting one or more of its surfaces (e.g., a rearsurface of the device, a front surface, etc.)) using measurements fromthe temperature sensors of the device, optionally in addition to thermalresistance values at various locations inside and/or outside the device.In some examples, the first absolute temperature sensor is disposed onor otherwise integrated with a first printed circuit board (PCB) (e.g.,a logic board, a main logic board, a system-in-package component, adisplay, etc.) and the second absolute temperature sensor is disposed onor otherwise integrated with a second PCB different from the first PCB.In some examples, estimating temperatures outside of the electronicdevice is a function of a first temperature measurement from the firstabsolute temperature sensor, a second temperature measurement from thesecond absolute temperature measurement, and one or more thermalresistance values corresponding to various locations or regions insideand/or outside the device. In some examples, an electronic deviceleverages measurements from a plurality of temperature sensors,including, for example, at least two temperature sensors (e.g., 2, 3, or4 temperature sensors), to measure and/or estimate ambient temperature.

FIGS. 1A-1G illustrate exemplary systems with heat flux and/ortemperature sensors configured for measuring or estimating temperatureswithin and/or outside the system according to some examples of thedisclosure. As described herein, thermopile temperature sensors measurea temperature differential (temperature gradient measurement)corresponding to a difference or gradient in temperature between twolocations within the electronic device. In some examples, systemsprovided with thermopile temperature sensors use temperature gradientmeasurements to estimate heat flux within the device. In some examples,the systems described herein can use measurements from thermopiletemperature sensors to estimate external temperatures (e.g., ambient airtemperature, skin temperature, etc.).

FIG. 1A illustrates an exemplary mobile telephone 136 that includes atouch screen 124 and can include a thermopile temperature sensing systemaccording to some examples of the disclosure. FIG. 1B illustrates anexample digital media player 140 that includes a touch screen 126 andcan include a thermopile temperature sensing system according to someexamples of the disclosure. FIG. 1C illustrates an example personalcomputer 144 that includes a touch screen 128 and a track pad 146, andcan include a thermopile temperature sensing system according to someexamples of the disclosure. FIG. 1D illustrates an example tabletcomputing device 148 that includes a touch screen 130 and can include athermopile temperature sensing system according to some examples of thedisclosure. In some examples, the thermopile temperature sensing systemwithin mobile telephone 136, digital media player 140, personal computer144 or tablet computing device 148 can be used to measure temperaturesassociated with touch screens 124, 126, 128 or 130, or with track pad146 (e.g., temperatures inside these devices, temperatures outside thesedevice, temperatures of surfaces contacting or in proximity to the touchscreen or track pad of these devices, etc.).

FIG. 1E illustrates an example wearable device 150 (e.g., a watch) thatincludes a touch screen 152 and can include a thermopile temperaturesensing system according to some examples of the disclosure. In someexamples, the thermopile temperature sensing system within wearabledevice 150 can be configured to measure temperatures associated withtouch screen 152. Wearable device 150 can be coupled to a user via strap154 or any other suitable fastener. FIG. 1F illustrates another examplewearable device 160 (or alternatively can be viewed as a side view ofwearable device 150), that can include a thermopile temperature sensingsystem according to some examples of the disclosure. Wearable device 160can include a front face 162, a housing 164 and a back face 166. Thefront face 162 is sometimes referred to herein as the “front crystal” ofwearable device 160 and the back face 166 is sometimes referred toherein as the “back crystal” of wearable device 160. However, it shouldbe understood that the front face 162 and back face 166 generally referto a substrate such as glass, plastic, or crystal. For example, thefront face 162 and back face 166 (also referred to as a rear face) canprotect internal components of wearable device 160, but also allow foroptical transmission from a display screen (e.g., touch screen) and/oroptical sensors. In some examples, the thermopile temperature sensingsystem within wearable device 160 can be used to measure temperaturesassociated with a touch screen located at front face 162. In suchexamples, the thermopile temperature sensing system can be configured tomeasure the temperature at a location or region 163 inside wearabledevice 160 (e.g., optionally closer to the front face 162 than alocation of an absolute temperature sensor). Alternatively, oradditionally, the thermopile temperature sensing system can be used toestimate the temperature outside of wearable device 160, such as thetemperature of air contacting front face 162, or the temperature ofother objects at least partially in contact with, or overlapping, frontface 162. In some examples, the thermopile temperature sensing systemwithin wearable device 160 can used to measure temperatures associatedwith an optical system located at back face 166. In such examples, thethermopile temperature sensing system can be configured to measure thetemperature at a location or region 165 inside wearable device 160(e.g., optionally closer to the back face 166 than a location of anabsolute temperature sensor). Alternatively, or additionally, thethermopile temperature sensing system can be used to estimate thetemperature outside of wearable device 160, such as the temperature ofair contacting back face 166, or the temperature of other objects atleast partially in contact with, or overlapping, back face 166 (e.g.,skin temperature at the wrist). Finally, the thermopile temperaturesensing system can be used to estimate the temperature at any locationwithin wearable device 160, as well as the temperature of objectsoutside wearable device 160 (e.g., the air surrounding wearable device160, or objects at least partially in contact with front face 162, rearface 166, or housing 164).

FIG. 1G illustrates another example wearable device, in-ear headphones170, that can include thermopile temperature sensing system according tosome examples of the disclosure. In some examples, the thermopiletemperature sensing system within in-ear headphones 170 is used tomeasure temperatures of components within in-ear headphones 170, such astemperatures associated with circuitry within in earbud 172 orprotrusion 174. Alternatively, or additionally, the thermopiletemperature sensing system can be used to estimate the temperatureoutside of in-ear headphones 170, such as the temperature of aircontacting earbud 172, or protrusion 174, and the temperature of otherobjects at least partially in contact with, or overlapping, earbud 172,or protrusion 174 (e.g., ear/body temperature).

It should be understood that the example devices illustrated in FIGS.1A-1G are provided by way of example, and other types of devices caninclude a thermopile temperature sensing system for detectingtemperatures within or outside the devices. For example, the devices caninclude devices worn on or placed into contact with the face, the head,or the fingers of a user (or at another location on a user's body. Thedevices can include over-ear headphones, glasses, head bands, cheststraps, wrist straps, rings, etc. For example, glasses worn on a user'sface can include a thermopile temperature system to estimate skintemperature at a user's temples, forehead or nose, among otherpossibilities. In some examples, the glasses can include one or moreabsolute temperature sensor and one or more thermopiles terminating ator near locations of interest for temperature measurement (e.g., onethermopile can be used to estimate the temperature at a left temple anda second thermopile can be used to estimate temperature at a righttemple). In a similar manner, a head band, chest strap or ring caninclude one or more absolute temperature sensors and/or thermopiles tomeasure skin temperature at a location of contact with the user's body.In some examples, the thermopile temperature sensing system can be usedin a thermostat device (e.g., a wall mounted thermostat) or can beincorporated into a device to add a thermostat capability (e.g.,incorporating the thermopile temperature sensing system in a computer,tablet, media player, smart phone, smart speaker, etc. Additionally,although some of the devices illustrated in FIGS. 1A-1G explicitly referto touch screens, it is understood that the thermopile temperaturesensing system described herein does not require a touch screen.

As described herein, thermopile temperature sensing systems can beincorporated into the systems (e.g., illustrated in FIGS. 1A-1G) to addinternal and/or external temperature sensing capabilities to electronicdevices. In particular, the use of a thermopile as described herein canenable more accurate estimates because thermopiles can reduce the impactof thermal aggressors (e.g., heat sources within a device, such as heatgenerating components within the device) and can also reduce the overalldrift or other error in a temperature estimate (e.g., process, voltage,and/or temperature variations in the temperature sensors, influence fromthermal aggressors, etc.). For example, thermopiles can be viewed astwo-ended (or two-sided) devices, that output a temperature differenceor gradient measurement corresponding to the difference in temperatureof a first end (or side) of the thermopile relative to a second end (orside) of the thermopile. Thermopile temperature sensing systems cantherefore be configured such that they measure a temperature gradient ordifferential between the inner surface of a device, and another locationinside the device. As a result, placement of the absolute temperaturesensors can be separated from a location or region of temperaturemeasurement. Instead the absolute temperature sensors can be placed in afirst location and a thermopile can be used to estimate temperature asecond, different location. Thus, the absolute temperature sensor can beintegrated into a device away from temperature aggressors and/or provideflexibility for integration of the temperature sensors at a locationaway from a location or region of interest. Specifically, thethermopiles can have a first end coupled to a first location inside thedevice (e.g., proximity to the absolute temperature sensor), and canhave a second end coupled to a second location inside the device (e.g.,at a location or region of interest). Based on the temperature gradientmeasurement of such a thermopile, the temperature of the second locationat the second end of the thermopile can be estimated. Additionally,using an absolute temperature sensor and thermopile temperature sensorto measure heat flux as compared with using a pair of absolutetemperature sensors for a heat flux measurement can introduce the driftor other error of one absolute temperature sensor, rather than the driftor other error of two absolute temperature sensors. Moreover,integrating a thermopile into an electronic device may provide fortemperature estimation at a location of the device in which integrationof an absolute temperature sensor can be difficult or impossible to duespace constraints, particularly in devices that can be densely filledwith circuitry and therefore cannot easily accommodate additionalsensing devices.

Furthermore, the temperature of objects (e.g., air, a user, etc.) can beestimated at the second location or contacting or in proximity to alocation outside the device, opposite a corresponding second locationinside the device. In some examples, the thermopile temperature sensingsystems estimate external temperature continuously during the operationof a device. For example, a system including the thermopile temperaturesensing system described herein can be used to measure a user's bodytemperature and/or track a user's body temperature when authorized to doso by a user. In particular, wearable electronic devices in proximitywith a user's body throughout a day and/or or night can provide forseamlessly measuring body temperature. In some examples, the type ofbody temperature estimate may be change depending on use conditions,such as time of day or user physiological characteristics. For example,when a user is vasoconstricted (e.g., limbs and extremities receive lessblood flow), a temperature measurement of a user's skin at the wristusing a wrist-worn electronic device may not accurately reflect theuser's core body temperature. In some examples, a wearable electronicdevices are worn on a user's wrist (or other limbs or extremities), canbe used to estimate physiological temperature values at night, when auser can be less likely to be vasoconstricted. Other wearable electronicdevices that can be worn around the chest, on the head, over the eyes,or even positioned within an opening of the body, can be used toestimate physiological temperature values at any time. In some examples,a back face of a wearable electronic device can be used to estimate bodytemperature (e.g., by estimating wrist temperature) when the user can bein a vasodilation condition (e.g., such as at night when a user sleeps),but a user can measure temperature at a region of the body different ator closer to core body temperature using a front face of the electronicdevice to enable measurements even during vasoconstriction. For example,a user may bring the front face of a wearable device into contact withthe forehead to measure core body temperature.

FIG. 2 illustrates a block diagram of a computing system of an exemplaryelectronic device that includes a temperature sensing system accordingto some examples of the disclosure. Although primarily described hereinas a wearable device, the computing system may alternatively beimplemented partially or fully in a non-wearable device. For example,the sensors and/or processing described herein can be implementedpartially or fully in a mobile telephone, media player, tablet computer,personal computer, server, etc. In some examples, the optical sensors(e.g., light emitters and light detectors) and/or temperature sensors(e.g., absolute temperature sensor(s) or heat flux sensors) can beimplemented in a wearable device (e.g., a wristwatch) and the processingof the optical and/or temperature data can be performed in anon-wearable device (e.g., a mobile phone). In some examples, thetemperature sensors, such as, can be implemented in a wearable device,and the processing of the data can be performed in a non-wearabledevice. Processing and/or storage of the optical and/or temperature datain a separate device can enable the device including the physiologicalsensors (e.g., a wristwatch) to be space and power efficient (which canbe important features for portable/wearable devices).

Computing system 200 can correspond to mobile telephone 136, mediaplayer 140, personal computer 144, tablet computer 148, wearable device150, wearable device 160, or in-ear headphones 170 above illustrated inFIGS. 1A-1G (or may be implemented in other wearable or non-wearableelectronic devices). Computing system 200 can include a processor 210(or more than one processor) programmed to (configured to) executeinstructions and to carry out operations associated with computingsystem 200. For example, using instructions retrieved from programstorage 202, processor 210 can control the reception and manipulation ofinput and output data between components of computing system 200.Processor 210 can be a single-chip processor (e.g., an applicationspecific integrated circuit) or can be implemented with multiplecomponents/circuits. For example, FIG. 2 illustrates that processor 210can include a relatively lower power processor 211-1 and a relativelyhigher power processor 211-2, as described in more detail herein.

In some examples, processor 210 together with an operating system canoperate to execute computer code, and produce and/or use data. Thecomputer code and data can reside within a program storage 202 that canbe operatively coupled to processor 210. Program storage 202 cangenerally provide a place to hold data used by computing system 200.Program storage block 202 can be any non-transitory computer-readablestorage medium. By way of example, program storage 202 can includeRead-Only Memory (ROM), Random-Access Memory (RAM), hard disk driveand/or the like. The computer code and data could also reside on aremovable storage medium and loaded or installed onto computing system200 when needed. Removable storage mediums include, for example, CD-ROM,DVD-ROM, Universal Serial Bus (USB), Secure Digital (SD), Compact Flash(CF), Memory Stick, Multi-Media Card (MMC) and/or a network component.

As described herein, in some examples, host processor 210 can representmultiple processors, such as lower power processor 211-1 and higherpower processor 211-2. Lower power processor 211-1 and higher powerprocessor 211-2 can represent separate processing chips, each withindependent timing and power requirements. For example, lower powerprocessor 211-1 can operate using a first clock signal and at a firstpower level that allows processor 211-1 to remain operational (“on”)across most or all operating modes of system 200 (e.g., a sleep mode,awake mode, idle mode, etc.). By contrast, higher power processor 211-2can operate using a second clock signal (e.g., a higher frequencyclock), different from the first, and at a second power level, higherthan the first. Because of the higher power requirements of higher powerprocessor 211-2, host processor 210 (e.g., an operating system onprocessor 210) can selectively disable, or power down higher powerprocessor 211-2 or otherwise throttle its power consumption duringcertain operating modes of system 200 (e.g., a power saving mode, sleepmode, etc.). In some examples, as described herein, the higher powerprocessor 211-1 can be powered down or otherwise throttle its powerconsumption to enable temperature measurements without error introducedby the power dissipation by higher power processor 211-1.

Lower power processor 211-1 and/or higher power processor can interfacewith various sensors of system 200 including a touch sensor panel and/ora touch screen 220 (via touch and display controller 216), motion and/ororientation sensor(s) 230, optical sensor(s) 211 (via optical sensorcontroller 212), and temperature sensor(s) 250 (via temperature sensorcontroller 240). In some examples, lower power processor 211-1 canoperate in a sleep mode or a power-saving mode, while higher powerprocessor 211-2 is powered down. In some examples, lower power processor211-1 can change an operating mode of system 200 or otherwise causehigher power processor 211-2 to be powered on (e.g., when wake upconditions are detected).

Computing system 200 can also include power management circuitry 209and/or power dissipation monitoring circuitry 213. Host processor 210(e.g., lower power processor 211-1 and/or higher power processor 211-2)can be coupled to power management circuitry 209 and/or powerdissipation monitoring circuitry 213. Power management circuitry 209 canregulate power delivery from power supply circuitry (e.g., a battery, orother power source of system 200) to various components of system 200(e.g., sensors, processors, antennas, displays, etc.). As an example,power management circuitry 209 can interrupt or throttle power deliveryto components that generate heat within system 200 (e.g., thermalaggressors), especially during temperature measurements that may besensitive to heat from such components. Power management circuitry 209can monitor temperatures inside a housing of system 200 and/ortemperatures outside the housing (e.g., environmental temperatures, userskin/core temperature). As an example, power management circuitry 209can monitor these temperatures to detect unsafe operating conditions forsystem 200, and can selectively interrupt or throttle power delivery tocertain heat-generating components to bring system 200 into a safeoperating condition. In some examples, power management circuitry 209provides control signals to inline switches coupled between the powersupply circuitry of system and various components of system 200, wherethe control signals determine an amount of current or power that can bedelivered to the respective components. As an example, power managementcircuitry 209 can provide a first control signal to a switch interposedbetween a battery power source of system 200 and touch screen 220, suchthat the first control signal limits the amount of power or currentdelivered to the touch screen by the battery power source. As anotherexample, power management circuitry 209 can provide a second controlsignal to a switch interposed between a battery power source of system200 and antenna circuitry (not shown) of the system, such that thesecond control signal interrupts power delivery or current flow betweenthe battery power source and the antenna circuitry.

Power dissipation monitoring circuitry 213 can monitor power supplycircuitry of system 200 (not shown), and can regulate power deliveryfrom the power supply circuitry to various components of system 200(e.g., by sending instructions to power management circuitry 209). Insome examples, power dissipation monitoring circuitry 213 includes asensor coupled to the power supply circuitry (e.g., battery) of system200. The sensor can measure power drawn by components of system 200 fromthe power supply circuitry (e.g., battery of system 200). In someexamples, the power drawing by components of the system can be estimatedbased on a current draw from the power supply circuitry. In someexamples, the power drawn can be estimated on a device basis (e.g.,estimated current draw from the battery). In some examples, the powerdrawn can be estimated on a per-component basis for some (e.g., knownthermal aggressors) or all of the components. In some examples, thepower dissipation monitoring circuitry 213 includes at least oneresistor (e.g., with a resistance greater than 10 MOhm, 20 MOhm, etc.)coupled between with the power supply circuitry or battery of system 200and components of system 200 that draw power. A current through theresistor can be measured by determining a voltage across the resistor(e.g., periodically or in response to a trigger) and converting thevoltage to a resistance (e.g., using Ohms law).

In some examples, computing system 200 (e.g., processor 210, powermanagement circuitry 209, and/or power dissipation monitoring circuitry213) can include power dissipation models that relate current/power drawfrom the power supply or battery of system 200 and temperature or heatdissipation within the device. Additionally or alternatively, computingsystem 200 can include models for estimating the power consumptionand/or resulted temperature changes by different components, indifferent operational modes of system 200 (e.g., power consumption bytouch screen 220 in an idle mode, in a low-brightness mode, in ahigh-brightness mode, etc.). Impacts of the power consumption of certaincomponents, or thermal aggressors of system 200, can be determined usinglab characterizations of the components (e.g., a rise time, a fall time,and amplitude measured for each thermal aggressor at various respectivepower levels). Accordingly, computing system 200 can dynamically modeltemperatures within the system 200, based on power dissipation models,and one or more current/power draw measurement at the system's powersupply circuitry or battery. In some examples, power managementcircuitry 209 can limit or interrupt the delivery of power to certaincomponents, such as during a measurement interval associated withtemperature sensors 250 (e.g., an interval where sensor data iscollected from temperature sensors 250), based on information from powerdissipation monitoring circuitry 213. As an example, when a powerdissipation model indicated that an amount of power being drawn bycomponents of system 200 corresponds to a temperature within the deviceoutside of a range required for accurate and/or reliable operation oftemperature sensors 250, power management circuitry 209 to limit orinterrupt power to components of system 200 such that the total powerdrawn by the components can be reduced to a level corresponding to atemperature within the range required for accurate and/or reliableoperation of temperature sensors 250. In some examples, powerdissipation monitoring circuitry 213 and/or power management circuitrycan cause host processor 210 to delay the performance of certainfunctions or operations to limit or interrupt power to components ofsystem 200. As an example, host processor 210 can postpone operations(or modify operations for reduced power consumption) involving touchscreen 220, GPS circuitry (not shown), wireless communication chips (notshown), antennas (not shown), or other components of system 200 that canbe thermal aggressors, until after a measurement interval associatedwith temperature sensors 250 (e.g., an interval during which one or moreof the components receives less power).

Additionally or alternatively, characterizations of the components(e.g., a rise time, a fall time, and amplitude measured for each thermalaggressor at various respective power levels) can be used fortemperature compensation. For example, host processor 210 can usetemperature compensation models to adjust sensor measurements or sensordata according to the temperature within the device or the temperaturecontribution of thermal aggressors (e.g., heat-generating components ofsystem 200). As an example, the amount of power draw by components ofsystem 200 can be measured by power dissipation monitoring circuitry213. The measured power draw can be used to correct for heat fromthermal aggressors within the device. In some examples, the compensationcan be applied when the power draw corresponds to a temperature changeoutside of a range required for accurate and/or reliable operation oftemperature sensors 250. Accordingly, a temperature compensation model(e.g., the temperature change corresponding to the amount of power drawnby the components) can be used (e.g., by temperature sensor controller240) to adjust sensor data from temperature sensors 250 to account forthe elevated temperature within the device caused by thermal aggressors.

Computing system 200 can also include one or more input/output (I/O)controllers that can be operatively coupled to processor 210. I/Ocontrollers can be configured to control interactions with one or moreI/O devices (e.g., touch sensor panels, display screens, touch screens,physical buttons, dials, slider switches, joysticks, or keyboards). I/Ocontrollers can operate by exchanging data between processor 210 and theI/O devices that desire to communicate with processor 210. The I/Odevices and I/O controller can communicate through a data link. The datalink can be a unidirectional or bidirectional link. In some cases, I/Odevices can be connected to I/O controllers through wirelessconnections. A data link can, for example, correspond any wired orwireless connection including, but not limited to, PS/2, UniversalSerial Bus (USB), Firewire, Thunderbolt, Wireless Direct, IR, RF, Wi-Fi,Bluetooth or the like.

Computing system 200 can include a temperature sensor controller 240operatively coupled to processor 210 and to one or more temperaturesensors 250. As described herein, in some examples, the temperaturesensor controller 240 can be coupled to optical sensor controller 212.The temperature sensors 250 can include one or more absolute temperaturesensors 254, one or more heat flux sensors 256, and correspondingsensing circuitry 252 (e.g., analog and/or digital circuitry to measuresignals at the sensors 254/256, provide processing (e.g., amplification,filtering, level-shifting), and convert analog signals to digitalsignals). As an example, the one or more absolute temperature sensors254 and one or more heat flux sensors 256 can be configured to measuretemperature at various locations within system 200, including at leastone location or region inside the wearable device different than alocation or region in which an absolute temperature sensor is disposedfor system 200. These temperatures and/or heat flux measurements can beused to measure temperature characteristics of the device under variousmodes of operation (e.g., to estimate when temperatures within a deviceare approaching unsafe or unsustainable levels), to estimate ambienttemperatures outside the device, or to estimate a physiological signalassociated with a user (e.g., a body temperature of the user)). Measuredraw data from the absolute temperature sensors 254, heat flux sensors256, and sensing circuitry 252 can be transferred to processor 210 (viatemperature sensor controller 240), and processor 210 can perform thesignal processing described herein to estimate internal or externaltemperatures and/or to estimate physiological signals (e.g., bodytemperature associated with the user). Processor 210 and/or temperaturesensor controller 240 can operate temperature sensors 250 to measuretemperature values associated with system 200, and to estimatetemperature values associated with the environment external to thesystem. In some examples, temperature sensor controller 240 can includesignal processor 242 to sample, filter, and/or convert (from analog todigital) signals generated by various temperature sensors 250, which canbe positioned at different locations within a housing for system 200.Signal processor 242 can be a digital signal processing circuit such asa digital signal processor (DSP). The analog data measured by thetemperature sensors 250 can be converted into digital data by an analogto digital converter (ADC). In some examples, and the digital data fromthe temperature sensors can be stored for processing in a buffer (e.g.,a FIFO) or other volatile or non-volatile memory (not shown) intemperature sensor controller 240. In some examples, data from thetemperature sensors are used as inputs to a heat model for the device,and used to estimate temperatures external to the housing of system 200(e.g., temperature of an object or user that contacts a portion of thedevice or an ambient temperature). In some examples, processor 210and/or temperature sensor controller 240 can store the raw data and/orprocessed information in memory (e.g., ROM or RAM) for historicaltracking or for future diagnostic purposes.

To accurately model the environment outside of system 200, in someexamples, absolute temperature sensors 254 and heat flux sensors 256 canbe used in conjunction. In certain examples, temperature sensorcontroller 240 can use measurements from multiple separate absolutetemperature sensors 254, ideally located at well-characterized locationswithin the housing of system 200, to estimate heat flux through thedevice (e.g., without one or more dedicated heat flux sensors). In someexamples, absolute temperature sensors can include a negativetemperature coefficient (NTC) temperature sensor, a resistancetemperature detector (RTD), or a diode based temperature sensor. A heatflux sensor 256, such as a thermopile temperature sensor, includesmultiple thermocouples coupled in series. Each thermocouple can includetwo (or more) different conductive materials, characterized by orotherwise associated with different respective Seebeck coefficients. Afirst end of a heat flux sensor 256 can include a first set of junctionsbetween the two different conductive materials, and a second end of theheat flux sensor 256 can include a second set of junctions between thetwo different conductive materials. When these two ends of a heat fluxsensor 256 can be positioned at respective first and second locationswithin system 200, the heat flux sensor 256 can generate a voltagesignal proportional to a temperature gradient or a temperaturedifference between the first and second locations within system 200.When one end of a heat flux sensor 256 is positioned close to, ormechanically coupled to a location or region within a housing for system200, temperature sensor controller 240 can use the temperature gradientgenerated by the heat flux sensor to estimate the temperature of objectsthat contact an outer surface location of system 200 that can correspondto where one end of the heat flux sensor 256 can be positioned insidethe device.

Computing system 200 can include an optical sensor controller 212operatively coupled to processor 210 and to one or more optical sensors211. The optical sensor(s) can include light emitter(s) 204, lightdetector(s) 206 and corresponding sensing circuitry 208 (e.g., analogcircuitry to drive emitters and measure signals at the detector, provideprocessing (e.g., amplification, filtering), and convert analog signalsto digital signals). As an example, light emitters 204 and lightdetectors 206 can be configured to generate and emit light into a user'sskin and detect returning light (e.g., reflected and/or scattered) tomeasure a physiological signal (e.g., a photoplethysmogram, or PPGsignal). The absorption and/or return of light at different wavelengthscan also be used to determine a characteristic of the user (e.g., oxygensaturation, heart rate) and/or about the contact condition between thelight emitters 204/light detectors 206 and the user's skin. Measured rawdata from the light emitters 204, light detectors 206 and sensingcircuitry 208 can be transferred to processor 210, and processor 210 canperform the signal processing described herein to estimate acharacteristic (e.g., oxygen saturation, heart rate, etc.) of the userfrom the physiological signals. Processor 210 and/or optical sensorcontroller 212 can operate light emitters 204, light detectors 206and/or sensing circuitry 208 to measure data from the optical sensor. Insome examples, optical sensor controller 212 can include timinggeneration for light emitters 204, light detectors 206 and/or signalprocessor 214 to sample, filter and/or convert (from analog to digital)signals measured from light at different wavelengths. Optical sensorcontroller 212 can process the data in signal processor 214 and reportoutputs (e.g., PPG signal, relative modulation ratio, perfusion index,heart rate, on-wrist/off-wrist state, etc.) to the processor 210. Signalprocessor 214 can be a digital signal processing circuit such as adigital signal processor (DSP). The analog data measured by the opticalsensor(s) 211 can be converted into digital data by an analog to digitalconverter (ADC), and the digital data from the physiological signals canbe stored for processing in a buffer (e.g., a FIFO) or other volatile ornon-volatile memory (not shown) in optical sensor controller 212. Insome examples, some light emitters and/or light detectors can beactivated, while other light emitters and/or light detectors can bedeactivated (by power management circuitry 209) to conserve power, forexample, or for time-multiplexing (e.g., to avoid interference betweenchannels). In some examples, processor 210 and/or optical sensorcontroller 212 can store the raw data and/or processed information inmemory (e.g., ROM or RAM) for historical tracking or for futurediagnostic purposes.

In some examples, some light emitters and/or light detectors haveoperation characteristics that vary based on the temperature of thelight emitters and/or light detectors. As an example, some lightemitters may output light at a wavelength that varies based on thetemperature of the light emitter. In some examples, optical sensorcontroller 212 and/or processor 210 (higher power processor 211-2 and/orlower power processor 211-1) can receive temperature informationassociated with the light emitter (e.g., from temperature sensorcontroller 240), and adjust the wavelength of the optical sensor and/orprocessing of signals associated with the light emitter and/or acorresponding light detector based on the received temperatureinformation. For example, an estimation of a physiologicalcharacteristic (e.g., oxygen saturation, heart rate) may be sensitive towavelengths of light used to measure optical signals. In some examples,the optical sensor controller 212 and/or processor 210 can use thereceived temperature information to estimate a wavelength of lightgenerated by the optical sensor and compensate the estimation of thephysiological characteristic based on the estimated wavelength of light.

Computing system 200 can also include one or more motion and/ororientation sensors 230, such as an accelerometer, a gyroscope, aninertia-measurement unit (IMU), etc. In some examples, the motion and/ororientation sensors 230 can include a multi-channel accelerometer (e.g.,a 3-axis accelerometer).

Computing system 200 can also include, in some examples, a touch anddisplay controller 216 operatively coupled to processor 210 and to touchscreen 220. Touch screen 220 can be configured to display visual outputin a graphical user interface (GUI), for example. The visual output caninclude text, graphics, video, and any combination thereof. In someexamples, the visual output can include a text or graphicalrepresentation of the physiological signal (e.g., a PPG waveform) or acharacteristic of the physiological signal (e.g., oxygen saturation,heart rate, etc.) Touch screen can be any type of display including aliquid crystal display (LCD), a light emitting polymer display (LPD), anelectroluminescent display (ELD), a field emission display (FED), alight emitting diode (LED) display, an organic light emitting diode(OLED) display, or the like. Processor 210 can send raw display data totouch and display controller 216, and touch and display controller 216can send signals to touch screen 220. Data can include voltage levelsfor a plurality of display pixels in touch screen 220 to project animage. In some examples, processor 210 can be configured to process theraw data and send the signals to touch screen 220 directly. Touch anddisplay controller 216 can also detect and track touches or near touches(and any movement or release of the touch) on touch screen 220. Forexample, touch processor 218 can process data representative of touch ornear touches on touch screen 220 (e.g., location and magnitude) andidentify touch or proximity gestures (e.g., tap, double tap, swipe,pinch, reverse-pinch, etc.). Processor 210 can convert the detectedtouch input/gestures into interaction with graphical objects, such asone or more user-interface objects, displayed on touch screen 220 orperform other functions (e.g., to initiate a wake of the device or poweron one or more components).

In some examples, touch and display controller 216 can be configured tosend raw touch data to processor 210, and processor 210 can process theraw touch data. In some examples, touch and display controller 216 canprocess raw touch data itself (e.g., in touch processor 218). Theprocessed touch data (touch input) can be transferred from touchprocessor 218 to processor 210 to perform the function corresponding tothe touch input. In some examples, a separate touch sensor panel anddisplay screen can be used, rather than a touch screen, withcorresponding touch controller and display controller.

In some examples, the touch sensing of touch screen 220 can be providedby capacitive touch sensing circuitry (e.g., based on mutual capacitanceand/or self-capacitance). For example, touch screen 220 can includetouch electrodes arranged as a matrix of small, individual plates ofconductive material or as drive lines and sense lines, or in anotherpattern. The electrodes can be formed from a transparent conductivemedium such as ITO or ATO, although other partially or fully transparentand non-transparent materials (e.g., copper) can also be used. In someexamples, the electrodes can be formed from other materials includingconductive polymers, metal mesh, graphene, nanowires (e.g., silvernanowires) or nanotubes (e.g., carbon nanotubes). The electrodes can beconfigurable for mutual capacitance or self-capacitance sensing or acombination of mutual and self-capacitance sensing. For example, in onemode of operation, electrodes can be configured to sense mutualcapacitance between electrodes; in a different mode of operation,electrodes can be configured to sense self-capacitance of electrodes.During self-capacitance operation, a touch electrode can be stimulatedwith an AC waveform, and the self-capacitance to ground of the touchelectrode can be measured. As an object approaches the touch electrode,the self-capacitance to ground of the touch electrode can change (e.g.,increase). This change in the self-capacitance of the touch electrodecan be detected and measured by the touch sensing system to determinethe positions of one or more objects when they touch, or come inproximity to without touching, the touch screen. During mutualcapacitance operation, a first touch electrode can be stimulated with anAC waveform, and the mutual capacitance between the first touchelectrode and a second touch electrode can be measured. As an objectapproaches the overlapping or adjacent region of the first and secondtouch electrodes, the mutual capacitance therebetween can change (e.g.,decrease). This change in the mutual capacitance can be detected andmeasured by the touch sensing system to determine the positions of oneor more objects when they touch, or come in proximity to withouttouching, the touch screen. In some examples, some of the electrodes canbe configured to sense mutual capacitance therebetween and some of theelectrodes can be configured to sense self-capacitance thereof.

Note that one or more of the functions described herein, includingestimating a temperature internal or external to an electronic deviceaccording to some examples of the disclosure, can be performed byfirmware stored in memory (or in program storage 202) and executed bytemperature sensor controller 240, optical sensor controller 212, touchand display controller 216 or processor 210. The firmware can also bestored and/or transported within any non-transitory computer-readablestorage medium for use by or in connection with an instruction executionsystem, apparatus, or device, such as a computer-based system,processor-containing system, or other system that can fetch theinstructions from the instruction execution system, apparatus, or deviceand execute the instructions. In the context of this document, a“non-transitory computer-readable storage medium” can be any medium(excluding signals) that can contain or store the program for use by orin connection with the instruction execution system, apparatus, ordevice. The computer-readable storage medium can include, but is notlimited to, an electronic, magnetic, optical, electromagnetic, infrared,or semiconductor system, apparatus or device, a portable computerdiskette (magnetic), a random access memory (RAM) (magnetic), aread-only memory (ROM) (magnetic), an erasable programmable read-onlymemory (EPROM) (magnetic), a portable optical disc such a CD, CD-R,CD-RW, DVD, DVD-R, or DVD-RW, or flash memory such as compact flashcards, secured digital cards, USB memory devices, memory sticks, and thelike.

The firmware can also be propagated within any transport medium for useby or in connection with an instruction execution system, apparatus, ordevice, such as a computer-based system, processor-containing system, orother system that can fetch the instructions from the instructionexecution system, apparatus, or device and execute the instructions. Inthe context of this document, a “transport medium” can be any mediumthat can communicate, propagate or transport the program for use by orin connection with the instruction execution system, apparatus, ordevice. The transport medium can include, but is not limited to, anelectronic, magnetic, optical, electromagnetic or infrared wired orwireless propagation medium.

FIG. 3 illustrates a schematic view for an exemplary configuration of athermopile device and a thermopile measurement circuit according to someexamples of the disclosure. Thermopile sensing system includes athermopile 310, and a thermopile measurement circuit (sensing circuitry)330, which can be implemented with an electronic device (e.g., thedevices illustrated in FIGS. 1A-1G).

Thermopile 310 can correspond to a heat flux sensor 256, and sensingcircuitry 330 can include components from sensing circuitry 252 and/orsignal processor 242. Thermopile 310 can include a collection ofthermocouples connected in series, each thermocouple includingconductive segments of two different conductors. In general, any twodifferent conductors (or more than two conductors) can be used in athermopile, provided that they have different associated Seebeckcoefficients suitable to generate the desired temperature gradient. Asan example, conductive paths 314 of thermopile 310 can be formed from afirst conductive material such as copper (Cu), which has a first Seebeckcoefficient, and conductive paths 316 of thermopile 310 can be formedfrom a second conductive material such as copper-nickel alloy (CuNi,sometimes referred to as “constantan” for short, though the alloy maycontain a different ratio of materials than 55% copper-45% nickel),which has a second Seebeck coefficient that is different from the firstSeebeck coefficient. These specific materials used in conductive paths314 and 316 are exemplary, and any conductors with suitable differentSeebeck coefficients can be used in thermopile 310. Additionally, it isunderstood that although primarily described herein as using twodifferent conductors with two different Seebeck coefficients thatthermopiles can be implemented with more than two conductors withadditional Seebeck coefficients (e.g., three conductors, fourconductors, etc.). In some examples, the different materials can beselected to balance the positive and negative Seebeck effects for thetwo or more conductors used.

Conductive paths 314 can be coupled to conductive paths 316 in series ata two junctions. As described herein, the two junctions are oftenreferred to as hot junction 318 and cold junction 312, but it isunderstood that more generally these two junction can be referred to asa first junction and a second junction). As an example, a first path ofconductive paths 314 can extend from cold junction 312 to hot junction318, and can be coupled to a first path of conductive paths 316 at hotjunction 318 (e.g., by a via 319, or another connector). The first pathof conductive paths 314 and the first path of conductive paths 316 cancollectively form a first thermocouple. The first path of conductivepaths 316 can then extend from hot junction 318 to cold junction 312,and can be coupled to a second path of conductive paths 314 at coldjunction 312 (e.g., by a via 319, or another connector). The second pathof conductive paths 314 can then extend from cold junction 312 to hotjunction 318, and can be coupled to a second path of conductive paths316 at hot junction 318 (e.g., by a via 319, or another connector). Thesecond path of conductive paths 314 and the second path of conductivepaths 316 collectively form a second thermocouple that can be coupled inseries with the first thermocouple. Additional series thermocouples canbe formed using conductive paths 314 and 316 in a similar manner.

In some examples, conductive paths 314 can be formed on a differentlayer than conductive paths 316 (e.g., in a flexible printed circuit, orFPC), and can be coupled to each other using vias 319. For example, asshown conductive paths 314 and conductive paths 316 can be formed ondifferent layers (e.g., separated by a dielectric layer). As describedherein, in some examples, conductive paths 314 and conductive paths canbe implemented in a flexible printed circuit. In some examples,conductive paths 314 and conductive paths can be implemented in a rigidprinted circuit board. In other examples, conductive paths 314 can beformed in the same layer as conductive paths 316, separated by adistance, and connected using connectors (e.g., a conductive patternconnecting conductive paths 314 to conductive paths 316). In exampleswhere paths 314 and 316 can be formed on different layers, a respectiveconductive path of conductive paths 314 can partially overlap one ormore conductive paths of conductive paths 316 and/or a respectiveconductive path of conductive paths 316 can partially overlap one ormore conductive paths of conductive paths 314 (e.g., as shown in FIG. 3). For example, based on the example provided above, the first path ofconductive paths 316 can partially overlap the first path of conductivepaths 314 to enable a first via 319 therebetween, and the first path ofconductive paths 316 can partially overlap the second path of conductivepaths 314 (e.g., at an opposite junction) to enable a second via 319therebetween. Additionally, as described herein in more detail (e.g.,with respect to FIGS. 6A-10 ), in some examples, the thermopile can beimplemented using vias through a printed circuit board, with the viashaving different Seebeck coefficients.

Thermopile 310 can generate an output voltage between its two terminals315A and 315B at cold junction 312, corresponding to a temperaturedifferential or gradient between the two junctions (e.g., between hotjunction 318 and cold junction 312), thereby measuring heat flux throughits two ends (e.g., the hot and cold junctions) when the thermalresistance is known. In contrast to an arrangement where heat flux isestimated based on a differential between temperature readings from twodifferent absolute temperature sensors 254 whose position may beconstrained within the electronic device (e.g., the temperature sensorintegration may be difficult given space constraints and may be limitedto mounting on specific PCB s within the device at a distance from thelocation or region of interest), thermopile 310 can have its hotjunction 318 positioned directly at or closer to a temperature sensingsurface (e.g., locations or regions 163/165, of FIG. 1F), which canimprove coupling to the sensing surface of interest, as well asrepeatability of the heat flux measurements. Further, because thermopile310 directly measures temperature gradient, drift error that can bedoubled by the use of two absolute temperature sensors, which canprovide more accurate heat flux or temperature gradient measurement.

The use of a thermopile can also simplify manufacturing and increaseyield. For example, absolute temperature sensors can require extensivecharacterization and calibration (e.g., at manufacturing), to produceaccurate temperature readings within system 200. Thermopile 310,however, may not require factory calibration or may require less factorycalibration and characterization to produce accurate readings ascompared with an absolute temperature sensor, especially when used inconjunction with another absolute temperature sensor for heat fluxmeasurements. Thermopile 310 can have a higher accuracy than absolutetemperature sensors 254 (e.g., an order of magnitude improvement). As anexample, thermopile 310 may be accurate to +/−0.01 degrees Celsius oftemperature difference (ΔT), whereas a factory-calibrated andcharacterized absolute temperature sensor 254 may be accurate to +/−0.1degrees Celsius. As described herein, heat flux sensor 256 includingthermopile 310 can measure a temperature difference between twojunctions (e.g., cold junction 312 and hot junction 318), and one of thejunctions (e.g., cold junction 312) can be located adjacent to and canbe electrically connected to an absolute temperature sensor 254.

Based on both an absolute temperature measurement of sensor 254 locatedat cold junction 312, and the temperature difference or gradientmeasurement produced by thermopile 310, temperature sensor controller240 can estimate a temperature value at hot junction 318. Specifically,the temperature at hot junction 318 can be the temperature measured bysensor 254 located at the cold junction 312, plus the temperaturedifference or gradient measurement between hot junction 318 and coldjunction 312, generated by thermopile 310. In this manner, a thermopile310 can be positioned with its hot junction 318 at a surface or locationof interest, and temperature sensor controller 240 can measuretemperature at the location of interest using the temperaturedifferential/gradient value generated by thermopile 310 and an absolutetemperature sensor 254 located at or near cold junction 312. Anadvantage of such an arrangement over arrangements relying on absolutetemperature sensors, is enabling a thermopile can be formed with a hotjunction that is in direct contact, or more closely coupled to aparticular device surface (e.g., closer to front face 162 or rear face166 than an absolute temperature sensor) or component within the device(e.g., a light emitter 204 whose light emissions vary as a function oftemperature). Using a thermopile 310 enables temperature measurements atlocations of interest that may be otherwise impossible or impractical tocollect using an absolute temperature sensor (e.g., because the absolutetemperature sensor cannot be mounted or otherwise integrated as closewithin the device). As described herein, a thermopile can also provideadvantages over a thermocouple. For example, one advantage of athermopile can be a relatively high differential voltage across theseries thermocouple, which can provide a better signal-to-noise ratioand also can provide flexibility in selecting circuitry to sense thedifferential voltage. Another advantage of the thermopile can be thatthe differential can be averaged over an area of the thermopile ratherthan measuring a specific location, which can smooth out localizedtemperature anomalies.

Thermopile 310 can be connected to sensing circuitry 330, which caninclude a first stage 320, a second stage 322, and an analog-to-digitalconverter (ADC) 324. First stage 320 can include an amplifier 321 suchas an operational amplifier (op-amp). In some examples, the op-amp canhave a positive input terminal biased at a particular voltage level(e.g., using a resistor divider between power and ground or between ahigh and a low voltage), and a negative input that can coupled connectedan output of the op-amp. The output of the op-amp can be a bias voltageapplied to a first terminal 315A of thermopile 310 and coupled to thesecond stage 322 via an input resistance of the second stage 322. Insome examples, first stage 320 can be referred to as a bias amplifierbecause it generates a bias voltage for the thermopile 310 and for thesecond stage 322. Second stage 322 can include another amplifier 323(e.g., an op-amp), with an input resistance and a feedback resistance. Apositive terminal of the op-amp of second stage 322 can be connected toa second terminal 315B of thermopile 310. ADC 324 can be coupled tooutputs of first stage 320 and second stage 322. For example, the outputof the first stage can be used as a reference voltage and the output ofthe second stage can be used as an input voltage for the ADC. ADC 324can be configured to generate a digital value corresponding to thevoltage output by thermopile 310 (a differential operation that removesthe reference bias voltage from the output). In some examples, secondstage 322 can be referred to as a differential amplifier because itamplifies a differential voltage between the two terminals 315A-315B ofthermopile 310 (e.g., where the gain of the amplifier can be a functionof the feedback resistance). As mentioned above, the voltage output bythermopile 310, which includes multiple series-connected thermocouples,each optionally formed from conductive paths of metals with differentSeebeck coefficients (e.g., Cu, and CuNi), can be proportional to atemperature difference between cold junction 312 and hot junction 318.The digital conversion of the voltage output by first stage 320 andsecond stage 322, can be based on the voltage generated by thermopile310, and can be used by processing circuitry to estimate the temperatureat hot junction 318, based on an absolute temperature sensor measurementat or around cold junction 312 (e.g., the temperature at hot junction318 is the temperature at cold junction 312, plus the temperaturedifference between hot junction 318 and cold junction 312 generated bythermopile 310).

FIG. 4A illustrates a cross-sectional side view of an exemplaryelectronic device including one or more printed circuit boards andtemperature sensing circuitry according to some examples of thedisclosure. Wearable device 400 can correspond to a device 150 of FIG.1E and/or 160 of FIG. 1F (or more generally can correspond to any of theelectronic devices illustrated by FIGS. 1A-1G). Device 400 can include ahousing 410 secured to user 460 via a strap 412 or any other suitablefastener (e.g., corresponding to strap 154 and housing 164). In someexamples, device 400 can be secured to a user 460 (e.g., exposed skin onthe user's body). Device 400 can correspond to a watch, a fitnesstracker, or any other device (e.g., optionally used to measurephysiological signals associated with user 460). Device 400 can attachto user 460 around the wrist, head, over the eyes, or on any exposedsurface of the body that is suitable for measuring physiological signalsassociated with the user.

Multiple printed circuit boards (PCBs) 420, 430, and 440 are illustratedinside housing 410. For example, PCB 420 can be located inside device400, at a front face (sometimes referred to as a “front crystal”). Insome example, PCB 420 can be used to implement a touch sensor panel,display or touch screen (e.g., touch screen 220) disposed below thefront face. PCB 430 can be located inside device 400, between PCB 420and 440. In some examples, PCB 430 can include host processor 210,program storage 202, and optionally touch and display controller 216,optical sensor controller 212 and/or temperature sensor controller 240.In some examples, PCB 430 can also include a discrete absolutetemperature sensor 432 (e.g., similar to absolute temperature sensor 254of FIG. 2 ). PCB 440 can be located inside device 400, below PCB 430 ator in proximity to a back face 450 (sometimes referred to as a “backcrystal. In some examples, PCB 440 can additionally or alternativelyinclude a discrete absolute temperature sensor 442. In some examples,absolute temperature sensor 442 can be separated from back face 450 byPCB 440, and PCB 440 can be separated from housing 410 (e.g., withoutdirect contact with housing 410 due to the existence of one or moreintervening layers or an air gap). For example, PCB 440 can be separatedfrom the inner surface of back face 450, and sometimes a barrier and oneor more adhesive layers (not shown) can be positioned between PCB 440and back face 450. PCB 440 can include optical sensors 211 configured toemit light and detect light through back face 450 (e.g., light emittersand detectors mounted on the opposite side of PCB 440 as sensor 442). Itshould be understood that the number of PCBs, the number of temperaturesensors, and placement of PCBs and distribution of components betweenthe PCBs shown in FIG. 4A is representative and non-limiting. Forexample, fewer or more PCBs can be used, fewer temperature sensors canbe used (e.g., omitting either absolute temperature sensor 432 orabsolute temperature sensor 442), more temperature sensors can be usedthan in the illustrated exemplary device (e.g., as shown in FIG. 18 ),or the components of system 200 can be distributed differently acrossthe one or more PCBs.

In some examples, heat flux through device housing 410 can be calculatedusing discrete absolute temperature sensors 432 and 442. As an example,the temperature measured by sensor 442 can be subtracted from thetemperature measured by sensor 432 to determine a temperature differencebetween the absolute temperature sensors mounted to PCB 430 and PCB 440.This temperature difference can then be used to calculate heat fluxthrough the device, as well as estimating a temperature outside of thedevice (e.g., ambient air temperature at the back crystal or bodytemperature at back crystal 450). However, accuracy of such anarrangement can rely on a high thermal resistance between the pair ofabsolute temperature sensors 432 and 442, and further relies on anassumption of no thermal aggressors or heat sources between sensors 432and 442. Further, because of the distances that separate sensors 432 and442 from the front face of device 400 and the back face 450, thetemperature estimates of these surfaces based on sensors 432 and 442 areprone to errors caused by sensor drift (e.g., process, voltage,temperature or strain variations in absolute temperature sensors432/442), thermal aggressors, and inaccurate device characterizations,leading to unreliable or unrepeatable temperature estimates for surfacesof device 400, or the environment outside device 400 (e.g., temperatureestimates of user 460). Ideally, to measure temperature at back face450, an absolute temperature sensor would be coupled, or placed adjacentto back face 450. However, such an arrangement may not feasible or theintegration challenges may be difficult, due to space considerations forthe absolute temperature sensor and/or challenges in routing power andother signal connections from a printed circuit board to the absolutetemperature sensors.

FIG. 4B illustrates a schematic view for an exemplary configuration thatincludes multiple thermopile devices coupled according to some examplesof the disclosure. Each of the multiple thermopiles can be used tomeasure a temperature differential to enable temperature estimates atdifferent locations within or outside a device. The configuration ofFIG. 4B includes one or more emitter and detector pairs (e.g.,corresponding to light emitters 204 and light detectors 206) and PCB440. PCB 440 can represent a printed circuit board including opticalsensing circuitry (e.g., corresponding to sensing circuitry 252) and oneor more absolute temperature sensor(s) 442. In some examples, thevarious emitter/detector pairs E_(i)/D_(i) illustrated in FIG. 4B can beat a different location than PCB 440 and absolute temperature sensor442. Additionally, each of the various emitter/detector pairsE_(i)/D_(i) can be disposed in a different location, and temperature canvary locally such that different emitters/detector pairs may experiencedifferent temperatures within the device. In some examples,emitter/detector pairs E_(i)/D_(i) can be separated from the absolutetemperature sensor 442 by the PCB 440. For example, emitter/detectorpairs E_(i)/D_(i) can be mounted to a bottom side of PCB 440 andabsolute temperature sensor 442 can be mounted to a top side of PCB 440.However, to simplify the following discussion, it is assumed thatemitter/detector pairs E_(i)/D_(i) can be located at various locationswithin the device that may have a different local temperature thanmeasured by the absolute temperature sensor 442.

As described herein, emitters E_(i) can correspond to light emitters204, and can optionally include light emitting diodes (LEDs). LEDs, orother light emitters 204, can produce a very narrow band of visible ornon-visible light, with an associated centroid wavelength of that band.The centroid wavelength of an LED can change, or drift, based on atemperature of the LED (e.g., thermal drift). Due to space constraints,there may not be adequate space adjacent to emitters E_(i) to provide anabsolute temperature sensor 442 to locally measure its temperature.Without an accurate temperature measurement of emitters E_(i), thermaldrift can introduce errors in measurements by the optical sensors (e.g.,measurements of physiological signals or estimates of physiologicalconditions). In some examples, thermopiles can be used to locallymeasure temperature at an emitter, and the locally measured temperaturecan be used to compensate for their temperature-dependent centroidwavelength drift, and avoid inaccurate or unreliable optical sensor dataor downstream estimates of physiological signals. In some examples, thewavelength of light emitted by the emitter can be estimated based on thetemperature and the optical sensing circuitry can tune the driving ofthe emitter so that the light can be emitted in the correct narrow band.In some examples, the wavelength of light emitted by the emitter can beestimated based on the temperature and the optical sensing system (e.g.,signal processor 214) can compensate for the change in wavelength tobetter estimate a physiological signal or condition.

To improve the accuracy and reliability of optical devices on device400, printed circuits 481 and 483 that include respective thermopiledevices can measure temperature associated with emitters E_(i) fordifferent emitter/detector pairs. For examples, a first end of printedcircuit 481 can be coupled to PCB 440 and can be coupled to absolutetemperature sensor 442 on PCB 440. In some examples, the first end ofprinted circuit 481 can be bonded to PCB 440 in proximity to absolutetemperature sensor 442 (e.g., co-located with absolute temperaturesensor 442). A thermopile device 477 of printed circuit 481 (representedby multiple horizontal lines, representing series-coupled thermocouplesof the thermopile), can span a length of the printed circuit 481 betweenabsolute temperature sensor 442 and emitter E1. The thermopile device477 can be used to measure a temperature differential between its firstend (e.g., at absolute temperature sensor 442) and its second end (e.g.,at or in proximity to emitter E1) using from a differential signalacross signal lines 484 (e.g., corresponding to first and secondterminals 315A and 315B, of FIG. 3 ). Emitter E1 can receive drive andcalibration signals from the system (e.g., processor 210, optical sensorcontroller 212) and/or transmit optical signals from detector D1 overunidirectional or bi-directional lines 482 of circuit 481. In someexamples, printed circuit 481 can be a flexible printed circuit with afirst end bonded to PCB 440. The flexible printed circuit 481 can thenflexes such that its second end reaches emitter E1. In some examples,emitter E1 and detector D1 can be implemented on the flexible printedcircuit 481. In some examples, printed circuit 481 can represent a rigidPCB or can be integrated within PCB 440 (e.g., when emitter E1 islocated on the underside of PCB 440).

As shown in FIG. 4B, in some examples, a second printed circuit 483 canbe used to estimate temperature to one or more additionalemitter/detector pairs E_(i)/D_(i). For example, FIG. 4B shows printedcircuit 483 including thermopiles that can be used to estimate atemperature at emitter/detector pair E2/D2 and/or at emitter/detectorpair E3/D3. A first end of printed circuit 483 can be coupled toabsolute temperature sensor 442/PCB 440 in a similar manner as describedfor printed circuit 481. Printed circuit 483 can split into multipletabs 483-1 and 483-2 at a point along its length. A first thermopiledevice 477A of printed circuit 483 (represented by multiple horizontallines, representing series-coupled thermocouples of the thermopile) canspan a length of the printed circuit 483 to an end of tab 483-1 (e.g.,between absolute temperature sensor 442 and emitter E2). The firstthermopile device 477A can be used to measure a temperature differentialbetween its first end (e.g., at absolute temperature sensor 442) and itssecond end at tab 483-1 (e.g., at emitter E2). A differential signal canbe measured from lines 494-1. A second thermopile device 477B of printedcircuit 483 can span a length of the printed circuit 483 to an end oftab 483-1 (e.g., between absolute temperature sensor 442 and emitterE3). The second thermopile device 477B can be used to measure atemperature differential between its first end (e.g., at absolutetemperature sensor 442) and its second end at tab 483-2 (e.g., atemitter E3). A differential signal can be measured from lines 494-2

Emitters E2 and E3 can receive drive and/or calibration signals from thesystem (e.g., processor 210, optical sensor controller 212) and/ortransmit optical signals from detectors D2 and D3 (e.g., to opticalsensor controller 212) over unidirectional or bi-directional lines 492of circuit 483. In some examples, printed circuit 483 can be a flexibleprinted circuit, that has a first end at PCB 440, and flexes such thatone of its tab ends reaches emitter E2, and another of its tab endsreaches emitter E3. In some examples, the emitters E2/E3 and detectorsD2/D3 can be implemented on tabs 483-1 and 483-2 of a flexible circuit.⁻

It is understood that multiple thermopiles can be implemented using oneor more printed circuits. In some examples, each printed circuit caninclude one thermopile (e.g., like printed circuit 481 with thermopile477). In some examples, multiple thermopiles can be implemented on oneprinted circuit (e.g., like printed circuit 483 with tabs 483-1 and483-2 including thermopiles 477A-477B). It is understood that althoughFIG. 4B references PCB 440 and absolute temperature sensor(s) 442, thatthe temperature sensing system can be implemented at other locationwithin device 400.

As described herein, in some examples, physiological characteristics orsignals can be estimated based on light detected by one or moredetectors D_(i), based on the estimated wavelength of light emitted byone or more emitters E_(i). Host processor 210 or temperature sensorcontroller 240, for example, can measure or estimate temperature at oneor more emitters E_(i), to estimate a centroid wavelength of lightemitted by each respective emitter E_(i). Based on the temperatureestimate/measurement at a particular emitter E_(i), the system canadjust a driving parameter associated with the particular emitter E_(i)to compensate for any potential drift in the centroid wavelength of theparticular emitter, and thereby improve the accuracy and reliability ofphysiological characteristics or signals based on light detected by oneor more detectors D_(i). Alternatively, or additionally, the system cancompensate an estimation of the physiological characteristic or signalbased on the estimated centroid wavelength of light emitted by eachrespective emitter E_(i).

It should be noted that the wavelength estimation techniques disclosedherein can be applied to any LED/PD (e.g., light emittingdiode/photodetector components (e.g., any of the optical sensors 211and/or any of the components therein (e.g., the light emitter(s) 204),sensing circuitry 208, or the light detector(s) 206) that are located atany location in the device (e.g., at the crown of a wearable device, atthe front crystal module (FCM) (e.g., front crystal) or at the backcrystal module (BCM) or back crystal).

As described herein, heat flux can be modeled to enable an estimate of atemperature outside of the device using temperatures inside the device.FIG. 5A illustrates a simplified schematic view of a heat flux model foran electronic device, such as the device of FIG. 4A, relative to auser's body according to some examples of the disclosure. As describedabove in connection with device 400, two absolute temperature sensors432 and 442 can be positioned at different locations inside the device(e.g., on separate PCBs 430 and 440, respectively). Model 500 shows twoseparate nodes, corresponding to T₂ and T₁, representing absolutetemperatures at absolute temperature sensors 432 and 442 inside device400, respectively. Additionally, model 500 shows a resistance R₁₋₂between the T₂ and T₁ nodes. R₁₋₂ can represent the thermal resistancebetween the T₂ and T₁ nodes, or the thermal resistance between absolutetemperature sensors 432 and 442. Notably, model 500 assumes noadditional heat sources or thermal aggressors are disposed betweensensors 432 and 442. An additional resistance R_(AMB) between the T₂node and the T_(AMBIENT) node can represent the thermal resistancebetween sensor 432 and the area outside device 400 (e.g., the ambientair temperature outside/above device 400). In the simplified model ofFIG. 5A, R_(AMB) can represent the combined thermal resistance of anycomponents, including PCB 420 and portions of housing 410, thatintervene between absolute temperature sensor 432 and the ambient airabove and outside the front face of device 400. Another resistance,R_(WRIST), between the T₁ node and the T_(WRIST) node can represent thethermal resistance between sensor 442 and user 460, specifically theuser's wrist for a wrist-worn wearable device. In the simplified modelof FIG. 5A, R_(WRIST) can represent the combined thermal resistance ofany components, including PCB 440 and back face 450, that intervenebetween sensor 442 and user 460.

Using Fourier's law, heat flux can be expressed using the followingexpression:

$Q = {{- \frac{1}{R}}\left( {\Delta T} \right)}$

where Q can represent heat flux between two nodes, ΔT can represent atemperature gradient or difference between two nodes and R can representthe thermal resistance between the nodes (1/R can represent the bulkthermal conductivity). Using two absolute temperature sensors 432 and442, which generate respectively temperature measurements T₂ and T₁, thetemperature at the wrist of user 460 (sometimes represented by T_(WRIST)or T_(w)) can be expressed using the following expression:

$T_{w} = {T_{1} + {\frac{R_{WRIST}}{R_{1 - 2}}\left( {T_{1} - T_{2}} \right)}}$

The above expression provides a model by which a user's wristtemperature can be estimated, using the T₂ and T_(i) temperaturemeasurements generated by absolute temperature sensors 432 and 442,along with values for R_(WRIST) and R₁₋₂ that can be characterized orotherwise determined at design and/or factory calibration of device 400(e.g., during manufacturing). Due to the simplification of the model,the expression above assumes constant heat flux through each of the PCBs430 and 440 on which absolute temperature sensors 432 and 442 can bepositioned, and further assumes no additional heat sources in betweenabsolute temperature sensors 432 and 442. However, these assumptions maynot always valid during the operation of device 400, which may lead toinaccurate or unreliable wrist temperature measurements for user 460using the above expression derived from model 500. Further, the wristtemperature T_(w) in model 500 often corresponds to the surface skintemperature, which may depart for core body temperature duringvasoconstriction. Although the above expressions are focused onestimating temperature of a user's body, it should be understood thatthe expression can be modified to estimate ambient temperature instead.

FIG. 5B illustrates a linear model circuit equivalent of the componentswithin device 400 (or within device 1800 discussed in detailed laterbelow) that can be used to estimate a core temperature of a wrist of auser 460 (or a user 1860), using absolute sensor measurements frominside device 400 or device 1800. In some examples, core bodytemperature can be estimated by a sum of core temperature of a wrist andthe heat flux through the user's arm multiplied by the thermalresistance through the arm. During vasodilation (e.g., during sleep) thethermal resistance through the arm can approach zero such that the corewrist temperature can be a close approximation of core body temperature.As described above, two absolute temperature sensors 432 and 442 can bepositioned at different locations inside device 400 (e.g., on separatePCBs 430 and 440, respectively). Model 550 shows two separate nodes,corresponding to T₂ and T₁, representing temperature at absolutetemperature sensors 432 and 442 inside device 400, respectively.Additionally, model 550 shows a resistance R₁₋₂ between the T₂ and T₁nodes. R₁₋₂ can represent the thermal resistance between the T₂ and T₁nodes, or the thermal resistance between absolute temperature sensors432 and 442. Notably, model 550, similar to model 500, assumes noadditional heat sources or thermal aggressors between sensors 432 and442 and also assumes steady state (e.g., the back face of the device andthe user' skin are at steady state). Model 550 includes resistanceR_(2-FC) between the T₂ node and the T_(FC) node that can represent thethermal resistance between absolute temperature sensor 432 and the frontcrystal of device 400 (e.g., corresponding to the region within device400 between PCB 420 and the front crystal (e.g., the temperature at alocation or region adjacent to PCB 420). R_(2-FC) can represent thecombined thermal resistance of any components, including PCB 420 andportions of housing 410, that intervene between sensor 432 and the frontface or front crystal of device 400 (including additional components notillustrated in FIG. 4A). Resistance R_(FC-A) between the T_(FC) node andthe T_(A) node can represent the thermal resistance between the frontcrystal of device 400 and the ambient air outside the front face ofdevice 400. R_(FC-A) can represent the combined thermal resistance ofthe interface between the front face or front crystal of device 400 andthe ambient air above device 400.

Model 550 also includes resistance R_(1-BC) between the T₁ and T_(BC)nodes that can represent the thermal resistance between absolutetemperature sensor 442 and the back face 450. R_(1-BC) can represent thecombined thermal resistance of any components, including PCB 440,between absolute temperature sensor 442 and back face 450 (includingadditional components not illustrated in FIG. 4A). Resistance R_(CONT)between the T_(BC) node and a T_(s) node (corresponding to skintemperature of user 460) can represent the thermal resistance of theback face 450. R_(S-A) in parallel with the series connected R_(CONT),R_(1-BC), R₁₋₂, R_(2-FC), and R_(FC-A). R_(S-A) can represent thethermal resistance of air between the user's skin and the air abovedevice 400. Finally, R_(PHYS) between the T_(S) node and a T_(C) node(corresponding to a corrected skin temperature that can minimize theeffect of heat exchange between the skin (e.g., wrist) and the ambienttemperature) can represent a physiological thermal resistance associatedwith user 460.

Φ_(BC) shown in model 550 can correspond to a heat flux correctionfactor for the impact of ambient air temperature on skin temperature. Atemperature heat flux correction, Φ_(BC)R₀, can be expressed using thefollowing expression:

ϕ_(BC) *R ₀ =h ₀*(T ₁ −T ₂)+h ₁

where h₀ can represent a multiplier parameter from characterization ofdevice 400 and h₁ can represent a self-heating parameter fromcharacterization of device 400 and/or a calibration offset fromcharacterization of device 400. The estimated temperature of backcrystal can be expressed using the following expression:

T _(BC) =T ₁+α₀*(T ₁ −T ₂)+α₁

where a₀ can represent to a multiplier parameter from characterizationof device 400 and h₁ can represent a self-heating parameter fromcharacterization of device 400 and/or a calibration offset fromcharacterization of device 400. It should be noted that a₁ and h₁ can befunctions or constants. For example, different values of a₁ and h₁ canbe used for different power modes of the device 400, such as the powermodes discussed later in this present disclosure. Specific values for a₀and h₀ can be determined based on the R_(1-BC), R₁₋₂ resistance valuesdescribed above and a physiological constant, R₀. In some examples R₀can be determined through lab validations (e.g., empirical measurementsbased on a representative sample of users). In some examples, themultiplier parameters a₀ and h₀ can be expressed using the followingexpressions:

${a_{0} = \frac{R_{1 - {BC}}}{R_{1 - 2}}},{h_{0} = \frac{R_{0}}{R_{1 - 2}}}$

Specific values for a₁ and h₁ can represent the impact of self-heatingof device 400 on temperature measurements inside the device. In otherwords, a₁ and h₁ can represent or model the contribution of thermalaggressors (and other heat sources) to temperature measurements insidedevice 400, which in turn can impact estimates of temperatures outsidedevice 400. In some examples, it is desirable to minimize a₀ to be closeto zero. In some examples, a₀ is minimized by increasing (and/ormaximizing) R₁₋₂, such as by increasing a distance between T₁ and T₂and/or by inserting a thermally resistive material between T₁ and T₂,such as a foam or another thermally resistive substrate. In someexamples, a₀ is minimized by decreasing (and/or minimizing) R_(1-BC),such as by decreasing a distance between T₁ and R_(BC).

Based on values determined empirically (e.g., through measurement, labvalidation, characterization in factory, etc.) for the variousresistances and parameters mentioned above, a device 400 can be capableof estimating corrected skin temperature of the user, or T_(C) using thefollowing expression:

T _(C) ≈T _(BC)+ϕ_(BC) R ₀,where R ₀ =αR _(phys) +R _(contact)

where R_(phys)(e.g., R_(PHYS)) can represent a physiological thermalresistance associated with the user, a can represent a ratio of heatflow in the tissue of the user over heat flow in the device, which isoptionally related to a ratio of the heat flow that is lost around thedevice (e.g., from the skin to the ambient air), and R_(contact) (e.g.,R_(CONT)) can represent the contact resistance.

In some examples, such as the example of FIG. 4A, device 400 estimatesT_(BC) based on temperature measurements T₁ and T₂, generated by twoseparate absolute temperature sensors, such as sensors 432 and 442 inFIG. 4A. However, as described herein, each of the absolute temperaturesensors can have an associated error or drift, caused by variations inthe sensor manufacturing process, variations in a supply voltageprovided to the sensor, or variations in the operating temperature ofthe sensor. In some examples, as described herein, the error or driftcan be reduced by measuring T_(BC) using a thermopile 310 with one endcoupled to or otherwise configured measure the temperature at the backcrystal of device 400 (e.g., attached to back face 450). Specifically, ahot junction 318 of thermopile 310 can be secured directly to back face450 with adhesives (e.g., a patterned adhesive layer or conductiveepoxy), and a cold junction 312 can be coupled to an absolutetemperature sensor 254 (e.g., absolute sensor 432 or 442). In suchexamples, absolute temperature sensor 254 can generate a temperaturemeasurement at its own location within device 400, and thermopile 310can generate a temperature differential or gradient measurement betweenits hot junction 318 (e.g., secured to back face 450) and its coldjunction 312 (e.g., co-located with absolute temperature sensor 254).Temperature sensor controller 240 (or host processor 210) of FIG. 2 cancalculate the temperature at back face 450 by summing the absolutetemperature measurement from absolute temperature sensor 254 and thetemperature gradient measurement from thermopile 310 (e.g., atemperature difference between the hot junction and the cold junction ofthe thermopile).

Device 400 can utilize either of models 500 or 550 for estimating wristtemperature, (or alternatively ambient temperature). Model 550, however,may rely on fewer assumptions about the absence of self-heating elementswithin device 400 such as thermal aggressors. Additionally oralternatively, models 500 or 550 can also be used for devices includingmultiple absolute temperature sensors or a single absolute temperaturesensor in combination with a thermopile 310 (as described herein). Ingeneral, model 550 can be utilized and/or modified for otherarrangements as well, such as where a thermopile 310 is coupled to othersurfaces of housing 410 (e.g., a front face of the device 400 above PCB420 or to PCB 420, to directly measure T_(FC)). When estimating wristtemperature of a user 460 or estimating core temperature of the user'sbody, device 400 can maintain some user-specific parameters that varyfrom user to user. In some examples, different users can have differentphysiological resistance values, such as R_(CONT) and R_(PHYS), anddevice 400 can be calibrated by a user, or during manufacture, to relyupon values for physiological resistance that correspond to a particularuser. It should be noted that the models 500 and 550 can correspond tothe estimation of core body temperature mentioned in block 1416 of FIG.14 discussed below; also, another model (e.g., a similar model that ismodified with different resistor values (e.g., thermal resistancevalues) (and/or more or fewer, different resistors) based on thelocation of the temperature sensors, thermopile, noise aggressors (e.g.,internal thermal noise aggressor components of the device), etc. can beused and solved to estimate other temperatures, such as one or more ofthe temperatures described above with reference to the model 550.Although primarily described in the context of a thermopile (flexibleprinted circuit or rigid PCB), the techniques described herein aremodifiable and/or applicable to an electronic device including multipleabsolute temperature sensors without a thermopile (e.g., as describedwith reference to the model of FIG. 19 ).

FIG. 6A illustrates a cross-sectional side view of an exemplaryelectronic device with temperature sensing circuitry and/or heat fluxsensing circuitry integrated with a printed circuit board according tosome examples of the disclosure. Device 600 can correspond to device 400of FIG. 4A, with the exception of a barrier 610 between PCB 440 and backface 450. Barrier 610 can be a rigid, multi-layer PCB that can includean integrated thermopile (e.g., similar to thermopile 310 of FIG. 3 ).In particular, an integrated or embedded thermopile within barrier 610can be formed using vias, such as through-hole vias, blind vias, orburied vias as described with reference to FIGS. 7A-10 . PCB 440 canhave a bottom surface that faces barrier 610 and back face 450,illustrated in FIG. 6B. Barrier 610 can have a top surface that abuts,is adjacent to, or is attached to the bottom surface of PCB 440,sometimes using a patterned adhesive layer. Barrier 610 can also have abottom surface that abuts, is adjacent to, or is attached to back face450, sometimes using a patterned adhesive layer. Interposed between PCB440 and back face 450 in this way, barrier 610 can include an embeddedthermopile with a first junction (e.g., hot junction) formed at thebottom surface of barrier 610, and with a second junction (e.g., coldjunction) formed between the bottom surface and the top surface ofbarrier 610.

In some examples, an embedded thermopile can be formed within barrier610 using through-hole vias. In such examples, the embedded thermopilecan have a hot junction at the bottom surface of barrier 610 thatcontacts the back face 450, and a cold junction at the top surface ofbarrier 610 that either contacts PCB 440, or is otherwise coupled to PCB440. Absolute temperature sensor 442 located on PCB 440 can measure atemperature at a top surface of PCB 440. The temperature at the backface 450 (T_(BC)) can be estimated by adding the temperaturedifferential or gradient measurement generated by the embeddedthermopile of barrier 610 to an absolute temperature measurementgenerated by sensor 442. In other examples, an embedded thermopile canbe formed within barrier 610 using blind vias. In such examples, theembedded thermopile can have a hot junction at the bottom surface ofbarrier 610 that contacts the back face 450, and a cold junction belowthe top surface of barrier 610 that does not extend to PCB 440. Athermopile formed using blind vias may not span the entire thickness ofbarrier 610, but still produces a temperature differential or gradientmeasurement that temperature sensor controller 240 can use to estimatethe temperature at back face 450 (T_(BC)). In yet other examples, anembedded thermopile is formed within barrier 610 using buried vias. Insuch examples, the embedded thermopile has a hot junction above thebottom surface of barrier 610 that does not contact the back face 450,and a cold junction below the top surface of barrier 610 that does notextend to PCB 440. A thermopile formed using buried vias may not spanthe entire thickness of barrier 610, but still produces a temperaturedifferential or gradient measurement that temperature sensor controller240 can use to estimate the temperature at back face 450 (T_(BC)).

In some examples, the thermopile can be integrated with both PCB 440 andbarrier 610. In some examples, the embedded thermopile can have a coldjunction at a top surface of PCB 440 and a hot junction at a bottomsurface of barrier 610, with through-hole vias used to implement thethermopile through both PCB 440 and barrier 610. In some examples, thethermopile can be integrated some, all or different combinations of thelayers described herein for PCB 440 and/or barrier 610.

In general, barrier 610 can have a relatively high thermal resistance byvirtue of being a thick, rigid PCB. In some examples, the thermalresistance can be between 1-5 Kelvin/Watt for a PCB of 100 squaremillimeters that is 0.3-1 mm thick. It is understood that thermalresistance can be greater or less than the range above dependent on thearea, thickness and materials of the PCB. In some examples, thethickness of barrier 610 can be greater than 300 microns. In someexamples, the thickness of barrier 610 can be greater than 1 millimeter.In some examples, the thickness of the barrier can be less than 300microns (e.g., 100-300 microns). Because of the high thermal resistanceof barrier 610, and the reliable sensitivity of its embedded thermopile,barrier 610 may not require the same individual calibration andcharacterization required by absolute temperature sensors, such asabsolute temperature sensor 442. Any number of thermopiles can beembedded within barrier 610. In some examples, a single thermopile canbe used to measure a temperature gradient for a region. In someexamples, multiple thermopiles can be used to independently measure atemperature gradient a different locations. In this way, one or multipletemperature gradient or differential measurements corresponding to theregion of barrier 610 or different sub-regions of barrier 610 can begenerated, and used by temperature sensor controller 240 to measure thetemperature of various circuit components, such as circuit componentsmounted on the bottom surface of PCB 440 that are adjacent to barrier610.

FIG. 6B illustrates an exploded view of a portion of an exemplaryelectronic device including one or more printed circuit boards and anintegrated thermopile according to some examples of the disclosure. FIG.6B can show a relationship of various sensing assemblies to a back faceof the electronic device. Specifically, FIG. 6B shows the bottomsurfaces of PCB 440 and barrier 610 (with the direction inverted in FIG.6B relative to the direction of PCB 440 and barrier 610 in FIG. 6A). Asillustrated, multiple emitter-detector pairs (labeled E_(i)/D_(i)) canbe mounted at the bottom surface of PCB 440. These emitter-detectorpairs can correspond to light emitters 204 and light detectors 206, andcan be used to measure physiological signals, as described above inconnection with FIG. 2 . Although described as pairs, it should beunderstood that multiple light paths can be formed between a respectiveemitter and multiple detectors or between multiple emitters and arespective detector. Additionally, it is understood that the numberand/or arrangement of emitters and detectors in FIG. 6B is an example,and can be varied. In some examples, the wavelength of light emitted bylight emitters 204 can vary as a function of temperature, which canimpact an estimation of a physiological characteristic based on thedetected light by light detectors 206. Accordingly, in some examples,estimating the temperature at the light emitters 204 or in the regionincluded the light emitters can provide an estimation of the wavelengthof the light emitter(s). This estimation of wavelength can be used forcalibrating the emitted wavelength or compensating to results of lightmeasurement or physiological characteristic estimation, which canimprove the accuracy of physiological signals derived from suchmeasurements.

Barrier 610 is illustrated with cutouts 627 corresponding to thelocations of emitters and detector mounted at the bottom surface of PCB440. When assembled, cutouts 627 in barrier 610 allow for light to beemitted by the emitters and to be detected by the detectors.Additionally, barrier 610 can prevent or reduce cross-talk between lightemitters and light detectors. Vias of the thermopile can be representedby the unshaded circles adjacent to cutouts 627 in barrier 610. Thevisible surface of these vias on the bottom surface of barrier 610 cancorrespond to the hot junction of a thermopile embedded within barrier610. As an example, vias 622-E and 622-D can correspond to twothermopiles for measuring the temperature at emitter E1 and at detectorD1, respectively, and can optionally generate independent temperaturedifferential or gradient measurements associated with each of thecomponents. Alternatively, vias 622-E and 622-D can together correspondto a single thermopile for the emitter-detector pair E1/D1, and cangenerate a single temperature differential or gradient measurementassociated with the two adjacent components. In particular, thethermopile(s) corresponding to vias 622-E and 622-D can generate atemperature differential or gradient measurement between the bottomsurface of barrier 610 (e.g., the hot junction) and another surfacewithin barrier 610 that can include the top surface of barrier 610(e.g., when through-hole vias are used to form the thermopile).

The remaining vias illustrated in FIG. 6B can be substantially similarto vias 622-E and 622-D, and can similarly correspond to respectivecomponents mounted on PCB 440. As an example, vias 624-E can correspondto emitter E2, vias 624-D can correspond to detector D2, vias 626-E cancorrespond to emitter E3, vias 626-D can correspond to detector D3, vias628-E can correspond to emitter E4, and vias 624-D can correspond todetector D4. Any combination of vias illustrated in FIG. 6B can becoupled together to increase the number of thermocouples within athermopile, thereby improving the thermopile sensitivity, butpotentially reducing the number of potential independent measurements.Vias 634 in a region between cutouts 627 can be optional be formedwithin barrier 610 (e.g., between the rows of cutouts corresponding toemitter-detector pairs E_(i)/D_(i) on PCB 440).

In some examples, all of the vias illustrated in FIG. 6B are coupled toone another, forming a single thermopile embedded across differentregions of barrier 610 to measure the temperature differential acrossthe barrier 610. When vias are coupled to one another to form a singlethermopile, their corresponding hot and cold junctions can be formed atthe same respective layers within barrier 610, to ensure that any outputof the single thermopile corresponds to a temperature gradient ordifferential across a common distance/thickness within barrier 610. Thecold junction of a thermopile embedded within barrier 610 can beaccessible at the top surface of barrier 610 (not shown) can have twoterminals, similar to the description of FIG. 3 . When considered incombination with an absolute temperature measurement at or near thelocation of the cold junction (e.g., an output of sensor 442 on PCB440), the temperature differential or gradient measurement generated bythe thermopile within barrier 610 can be used to directly measure thetemperature at the optical sensor (e.g., to estimate wavelength) and/orto directly measure the temperature at back face 450 (T_(BC)), which canimproves the accuracy of estimates of wrist or core body temperatureassociated with user 460.

FIG. 7A illustrates a cross-sectional side view of an exemplarytwo-layer rigid printed circuit board with an integrated thermopile, andoptional circuit components mounted on top and/or bottom surfaces of thePCB according to some examples of the disclosure. Barrier PCB 700 (e.g.,optionally corresponding to barrier 610, but representative of anysuitable rigid PCB in the device) can be a two layer rigid board, with acore layer 704 between a top soldermask layer 702 and a bottomsoldermask layer 706. As described above in connection with thermopile310 of FIG. 3 , a thermopile can include a collection ofseries-connected thermocouples. Each thermocouple can correspond to ajunction between a first material with a first associated Seebeckcoefficient, and a second material with a second associated Seebeckcoefficient that is different from the first Seebeck coefficient. InFIG. 7A, a first material with a first Seebeck coefficient can berepresented with the letter “A,” and a second material with a secondSeebeck coefficient can be represented with the letter “B.” Forillustrative purposes, material A can be or include copper (Cu) (e.g.,an epoxy or resin with copper particles), and metal B can be or includea copper-nickel (CuNi) alloy such as constantan (e.g., an epoxy or resinwith copper and nickel particles or with constantan particles).

Conductive paths corresponding to the two different metals A and B, cancorrespond to conductive paths 314 and 316 of FIG. 3 , respectively. Tomanufacture the embedded thermopile shown inside barrier PCB 700, viaholes can be formed through core layer 704, and filled with vias 710 and712 (e.g., with material A and material B, respectively. Various PCBprocessing methods, using with various materials (e.g., high volumeproduction processes) can be used to create the vias. Deposition methodsfor forming the different layers include printing, dispensing materialinto holes/vias, and vacuum printing.

For illustrative purposes, via 710 can be filled with copper, copperparticles suspended in epoxy, or copper-coated particles suspended inepoxy (e.g., according to a “via fill” manufacturing process).Similarly, via 712 can be filled with constantan, constantan particlessuspended in epoxy, or constantan-coated particles suspended in epoxy.The two vias can be then connected using patterning at a copper layer ofbarrier PCB 700, such as copper connector 714 formed within thesoldermask layer 702, above core layer 704. Connecting vias 710 and 712using connector 714 forms a thermocouple, when the metal filled in via710 has a different Seebeck coefficient than the metal filled in via712. Since vias 710 and 712 span the thickness of core layer 704, athermopile formed from the series combination of such vias measures heatflux, or the temperature differential/gradient, across the thickness ofcore layer 704. As shown to the right of via 712, another pair of viassimilar to vias 710/712 are formed, and connected in series to via 712(e.g., using a copper connector below core layer 704). In this way, eachpair of connected vias with dissimilar conductors (e.g., metals) forms athermocouple, and connecting such pairs of connected vias forms athermopile embedded within barrier PCB 700.

A hot junction of the thermopile embedded within barrier PCB 700 can belocated at the conductive layer on one side of core 704 (e.g.,co-located with soldermask layer 702), and a cold junction of thethermopile embedded within barrier PCB 700 can be located at theconductive layer on the opposite side of core 704 (e.g., co-locatedsoldermask layer 706). Viewed in this manner, terminal 716A canillustrate a copper contact for a cold junction of the thermopileembedded within barrier PCB 700 and terminal 716B can illustrate acopper contact for a hot junction of the thermopile embedded withinbarrier PCB 700. As shown in FIG. 7A, optionally circuit components 720,722, and 724 can be surface-mounted on top and bottom surfaces ofbarrier PCB 700, along with the thermopile embedded within core layer704. In other words, the integration of the thermopile with the PCB asshown in FIG. 7A can still allow for other components to be mounted onthe PCB.

FIG. 7B illustrates a plan view of the exemplary rigid PCB with anintegrated thermopile of FIG. 7A, specifically corresponding to across-section at the A-A′ line of FIG. 7A. As shown in FIG. 7A, the A-A′line corresponds to a boundary between core layer 704 and soldermasklayer 706. In other examples, the illustration of FIG. 7B can correspondto a cross section at the boundary of soldermask layer 702 and corelayer 704. View 750 illustrates circular elements arranged in array thatcorrespond to vias comprising various materials. As an example, via752-A can include copper, while via 752-B can include constantan. Asshown in FIG. 7B, via 752-A and via 752-B can be connected, therebyforming a thermocouple. Similarly, via 754-A can include copper, whilevia 754-B can include constantan, and the two vias can be connected toform another thermocouple. A first thermocouple, formed from vias 752-Aand 752-B, can be connected to a second thermocouple, formed from vias754-A and 754-B, to form a thermopile. The first thermocouple and thesecond thermocouple can be connected on a opposite side of the carematerial in a layer not visible in view 750 (e.g., using a copperconnector 714). In this way, view 750 can illustrate a singlethermopile, when all the shown connected via pairs are interconnected onanother layer, in series. Alternatively, view 750 can illustratemultiple thermopiles, when different, independent subsets of theconnected via pairs are interconnected on another layer, in series.

Although FIGS. 7A and 7B illustrate and relate to a two-layer barrierPCB 700, a thermopile can be embedded, or formed within a barrier PCBwith more than two layers (e.g., an arbitrary number of layers).Specifically, two layers can be the minimum number required to form athermopile, and certain PCBs with more than two layers can also beprovided with an embedded or integrated thermopile.

FIG. 8 illustrates a cross-sectional side view of an exemplaryfour-layer rigid PCB with an integrated thermopile spanning four PCBlayers according to some examples of the disclosure. Barrier PCB 800 canbe a four-layer PCB with a soldermask layer formed on a top surface 802,and a soldermask layer formed on and a bottom surface 810. The fourlayer PCB can include a first core layer 804, a second core layer 806,and third core layer 808. The first core layer 804 can be disposed belowtop surface 802 of the soldermask layer, the second core layer 806 canbe disposed below the first core layer 804, and the third core layer 808can be disposed below the second core layer 806. In some examples, thefirst core layer 804, the second core layer 806 and/or the third corelayer 808 can be formed from the same material. Patterned conductivelayers can be disposed on surfaces of the core layers.

A first conductive material/first metal (e.g., copper) with a firstSeebeck coefficient can be represented with the letter “A,” and a secondconductive material/second metal (e.g., constantan) with a secondSeebeck coefficient can be represented with the letter “B.” Conductivepaths corresponding to the two different metals A and B, can correspondto conductive paths 314 and 316 of FIG. 3 , respectively. To manufacturethe embedded thermopile shown inside barrier PCB 800, via holes can beformed from soldermask layer 810 through to soldermask layer 802 (e.g.,a through via). Once formed, the vias holes can be filled with vias 811and 812 including metal A and metal B, respectively (e.g., as describedwith reference to FIG. 7A).

For illustrative purposes, via 811 can include copper, copper particlessuspended in epoxy, or copper-coated particles suspended in epoxy (e.g.,according to a “via fill” manufacturing process). Similarly, via 812 caninclude constantan, constantan particles suspended in epoxy, orconstantan-coated particles suspended in epoxy. The two vias can thenconnected using patterning at a copper layer of barrier PCB 800, such ascopper connector 814 formed within the soldermask layer 802, above firstcore layer 804. Interconnecting or coupling vias 811 and 812 usingconnector 814 can form a thermocouple, when the metal of via 811 has adifferent Seebeck coefficient than the metal of via 812. Because vias811 and 812 can span the thickness of core layers 808, 806, and 804, athermopile formed from the series combination of such vias can measureheat flux, or the temperature differential/gradient, across the entirethickness of core layers 808, 806, and 804. As shown to the right of via812, another pair of vias similar to vias 811/812 can be formed, andconnected in series to via 812. In this way, each pair of connected viaswith dissimilar metals can form a thermocouple, and connecting suchpairs of connected vias can form a thermopile embedded within barrierPCB 800.

A hot junction of the thermopile embedded within barrier PCB 800 can belocated at the conductive layer on one side of core 804 (e.g.,co-located with soldermask layer 802), and a cold junction of thethermopile embedded within barrier PCB 800 can be located at theconductive layer on the opposite side of core 808 (e.g., co-located withsoldermask layer 810). Viewed in this manner, terminal 816A canillustrate a copper contact for a cold junction of the thermopileembedded within barrier PCB 800 and terminal 816B can illustrate acopper contact for a hot junction of the thermopile embedded withinbarrier PCB 800. Although not shown in FIG. 8 , circuit components canbe surface-mounted on top and bottom surfaces of barrier PCB 800 alongwith the thermopile embedded within core layers 808, 806, and 804 in asimilar manner as described with reference to FIGS. 7A-7B. Further, insome examples, a region 850 (shown to the right of the embeddedthermopile) can be unoccupied by any thermocouple/thermopile structures,leaving room for circuit components such as component 820 to be embeddedwithin core layer 806. Relative to the two layer barrier PCB of FIG. 7A,the four layer barrier PCB of FIG. 8 can provide a relatively higherthermal resistance and improved sensitivity.

FIG. 9 illustrates a cross-sectional side view of an exemplaryfour-layer rigid PCB with an integrated thermopile spanning two PCBlayers (and partially spanning three layers) according to some examplesof the disclosure. Barrier PCB 900 can be a four-layer PCB with asoldermask layer formed on a top surface 902 and on a bottom surface910. The four layer PCB can include a first core layer 904, a secondcore layer 906, and third core layer 908. The first core layer 904 canbe disposed below top surface 902 of the soldermask layer, the secondcore layer 906 can be disposed below the first core layer 904, and thethird core layer 908 can be disposed below the second core layer 906. Insome examples, the first core layer 904, the second core layer 906,and/or the third core layer 908 can be formed from the same material.Patterned conductive layers can be disposed on surfaces of the corelayers.

A first conductive material/first metal (e.g., copper) with a firstSeebeck coefficient can be represented with the letter “A,” and a secondconductive material/second metal (e.g., constantan) with a secondSeebeck coefficient can be represented with the letter “B.” Conductivepaths corresponding to the two different metals A and B, can correspondto conductive paths 314 and 316 of FIG. 3 , respectively. To manufacturethe embedded thermopile shown inside barrier PCB 900, vias holes can beformed from soldermask layer 910 through to second core layer 906 (e.g.,blind via and buried vias). Once formed, the via holes can be filledwith vias 911 and 912 including metal A and metal B, respectively (e.g.,as described with reference to FIG. 7A).

For illustrative purposes, via 911 can include copper, copper particlessuspended in epoxy, or copper-coated particles suspended in epoxy (e.g.,according to a “via fill” manufacturing process). Similarly, via 912 caninclude constantan, constantan particles suspended in epoxy, orconstantan-coated particles suspended in epoxy. The two vias can thenconnected using patterning at a copper layer of barrier PCB 900, such ascopper connector 914 formed within the soldermask layer 902, abovesecond core layer 906. Interconnecting or coupling vias 911 and 912using connector 914 can form a thermocouple, when the metal of via 911has a different Seebeck coefficient than the metal of via 912. Becausevias 911 and 912 can span the thickness of core layers 908 and/or 906(e.g., primarily core layer 906 for buried vias), a thermopile formedfrom the series combination of such vias can measure heat flux, or thetemperature differential/gradient, across the thickness of core layers908, and 906, but not across the entire barrier PCB 900. As shown to theright of via 912, another pair of vias similar to vias 911/912 can beformed, and connected in series to via 912. In this way, each pair ofconnected vias with dissimilar metals can form a thermocouple, andconnecting such pairs of connected vias can form a thermopile embeddedwithin barrier PCB 900.

A hot junction of the thermopile embedded within barrier PCB 900 can belocated at the conductive layer on one side of core 904 (e.g.,co-located with second core layer 904), and a cold junction of thethermopile embedded within barrier PCB 900 can be located at theconductive layer on the opposite side of core 908 (e.g., co-located withsoldermask layer 910). Viewed in this manner, terminal 916A canillustrate a copper contact for a cold junction of the thermopileembedded within barrier PCB 900 and terminal 916B can illustrate acopper contact for a hot junction of the thermopile embedded withinbarrier PCB 900. Although not shown in FIG. 9 , circuit components canbe surface-mounted on top and bottom surfaces of barrier PCB 900 alongwith the thermopile embedded within core layers 906 and 908 in a similarmanner as described with reference to FIGS. 7A-7B. Further, in someexamples, a region 950 (shown to the right of the embedded thermopile)can be unoccupied by any thermocouple/thermopile structures, leavingroom for circuit components (not shown) to be embedded within core layer906, or other through-hole vias and blind vias (pictured) can be usedfor other purposes (e.g., routing signals, etc.). Relative to the twolayer barrier PCB of FIG. 7A, the four layer barrier PCB of FIG. 9 canprovide a relatively higher thermal resistance, and improvedsensitivity.

FIG. 10 illustrates a cross-sectional side view of an exemplaryfour-layer rigid PCB with an integrated thermopile spanning two PCBlayers according to some examples of the disclosure. Barrier PCB 1000can be a four-layer PCB with a soldermask layer formed on a top surface1002, and a soldermask layer formed on a bottom surface 1010. The fourlayer PCB can include a first core layer 1004, a second core layer 1006,and third core layer 1008. The first core layer 1004 can be disposedbelow top surface 1002 of the soldermask layer, the second core layer1006 can be disposed below the first core layer 1004, and the third corelayer 1008 can be disposed below the second core layer 1006. In someexamples, the first core layer 1004, the second core layer 1006, and/orthe third core layer 1008 can be formed from the same material.Patterned conductive layers can be disposed on surfaces of the corelayers.

A first conductive material/first metal (e.g., copper) with a firstSeebeck coefficient can be represented with the letter “A,” and a secondconductive material/second metal (e.g., constantan) with a secondSeebeck coefficient can be represented with the letter “B.” Conductivepaths corresponding to the two different metals A and B, can correspondto conductive paths 314 and 316 of FIG. 3 , respectively. To manufacturethe embedded thermopile shown inside barrier PCB 1000, via holes can beformed from soldermask layer 1010 through to third core layer 1008(e.g., a blind via). Once formed, the via holes can be filed with vias1011 and 1012 including metal A and metal B, respectively (e.g., asdescribed with reference to FIG. 7A).

For illustrative purposes, via 1011 can include copper, copper particlessuspended in epoxy, or copper-coated particles suspended in epoxy (e.g.,according to a “via fill” manufacturing process). Similarly, via 1012can include constantan, constantan particles suspended in epoxy, orconstantan-coated particles suspended in epoxy. The two vias can thenconnected using patterning at a copper layer of barrier PCB 1000, suchas copper connector 1014 formed within the soldermask layer 1002, abovethird core layer 1008. Interconnecting or coupling vias 1011 and 1012using connector 1014 can form a thermocouple, when the metal of via 1011has a different Seebeck coefficient than the metal of via 1012. Becausevias 1011 and 1012 can span the thickness of core layers 1008, athermopile formed from the series combination of such vias can measureheat flux, or the temperature differential/gradient, across thethickness of core layers 1008, but not across the entire barrier PCB1000. As shown to the right of via 1012, another pair of vias similar tovias 1011/1012 can be formed, and connected in series to via 1012. Inthis way, each pair of connected vias with dissimilar metals can form athermocouple, and connecting such pairs of connected vias can form athermopile embedded within barrier PCB 1000.

A hot junction of the thermopile embedded within barrier PCB 1000 can belocated at the conductive layer on one side of core 1008 (e.g.,co-located with soldermask layer 1002), and a cold junction of thethermopile embedded within barrier PCB 1000 can be located at theconductive layer on the opposite side of core 1008 (e.g., co-locatedwith second core layer 1006 or more specifically, at the boundary oflayers 1006 and 1008). Viewed in this manner, terminal 1016A canillustrate a copper contact for a cold junction of the thermopileembedded within barrier PCB 1000 and terminal 1016B can illustrate acopper contact for a hot junction of the thermopile embedded withinbarrier PCB 1000. Although not shown in FIG. 10 , circuit components canbe surface-mounted on top and bottom surfaces of barrier PCB 1000 alongwith the thermopile embedded within core layer 1008 in a similar manneras described with reference to FIGS. 7A-7B. Further, in some examples, aregion 1050 (shown to the right of the embedded thermopile) can beunoccupied by any thermocouple/thermopile structures, leaving room forcircuit components (not shown) to be mounted and connected within corelayer 1006, or through-hole vias and other blind vias (pictured) can beused for other purposes (e.g., routing signals, etc.).

FIG. 11 illustrates a cross-sectional side view of an exemplaryelectronic device with temperature sensing circuitry and/or heat sensingcircuitry integrated with a flexible circuit according to some examplesof the disclosure. Device 1100 is substantially similar to device 400 ofFIG. 4A, with the exception of a flexible printed circuit (FPC) 1110that can extend from a top surface of PCB 440 to back face 450. FPC 1110can be a flexible, multi-layer printed circuit, that can include athermopile similar to thermopile 310 of FIG. 3 , embedded within aportion of its layers. In particular, an embedded thermopile within FPC1110 can be formed using conductive paths printed on the layers of theFPC, with the conductive paths being connected using vias. In someexamples, the conductive paths printed on the layers of FPC 1110 canextend across its entire (or substantially the entire) length (e.g.,conductive paths can extend from PCB 440 to back face 450). PCB 440 canhave a top surface, onto which an absolute temperature sensor 442 can besurface-mounted. Preferably, a thermopile embedded within FPC 1110 canhave a cold junction at a first end that can be coupled to the topsurface of PCB 440 and sensor 442. A thermopile embedded within FPC 1110can have a hot junction that can abuts, can be adjacent to, or can beattached to back face 450, sometimes using a conductive epoxy thatimproves the thermal coupling between the thermopile hot junction andback face 450. In such a configuration, a thermopile embedded within FPC1110 can measure a temperature differential or gradient between backface 450 (coupled to its hot junction), and PCB 440 (coupled to its coldjunction). When absolute temperature sensor 442 and a cold junction ofthe thermopile embedded within FPC 1110 are co-located, the sum of anabsolute temperature measurement generated at absolute temperaturesensor 442 and the temperature differential measured by the thermopilecan estimate the temperature at back face 450 (T_(BC)).

In general, embedding a thermopile within FPC 1110 can allow for directtemperature sensing/measurement at surfaces or regions inside device 400that are not practical to measure or estimate using an absolutetemperature sensor 254. As mentioned above in connection with FIG. 2 ,absolute temperature sensors can require supply power and otherconnections provided on a PCB, and therefore cannot be well-integratedto certain surfaces, such as the inner surface of back face 450).Moreover, a thermopile embedded within FPC 1110 can measure heat fluxacross its entire length (or substantially across its entire length),which can lead to temperature measurements at various surfaces that havegreater accuracy and repeatability, compared to estimating those surfacetemperatures using multiple absolute temperature sensors. Depending onthe number of conductive paths (sometimes referred to as “turns”) of athermopile within FPC 1110, the thermopile can have better sensitivitythan an absolute temperature sensor.

Although FPC 1110 with an embedded thermopile is illustrated asextending from sensor 442 to back face 450, the cold and hot junctionsof the embedded thermopile can be coupled between any two locationsinside device 400 (e.g., from PCB 430 to the back face 450, from PCB 430to PCB 440, from PCB 430 to PCB 420, etc.). In some examples, theembedded thermopile within FPC 1110 can have a cold junction coupled toa PCB including an absolute temperature sensor (e.g., one of absolutetemperature sensors 432/442) and a hot junction coupled to any otherlocation inside device 400 (e.g., back face 450, PCB 420, a sidewallinner surface of housing 410, etc.). In some examples, a hot junction ofthe embedded thermopile within FPC 1110 can be coupled to or inproximity to a circuit component, such as a light emitter 204 that emitslight with a wavelength that varies as a function of temperature. Insome examples, FPC 1110 and its embedded thermopile is coupled betweenabsolute temperature sensors 432 and 442. In some examples, FPC 1110 andits embedded thermopile can be coupled between absolute temperaturesensor 432 and PCB 420. In some examples, FPC 1110 and its embeddedthermopile can be coupled between absolute temperature sensor 442 andPCB 420. In some examples, FPC 1110 and its embedded thermopile can becoupled between one of absolute temperature sensors 432/442 and thefront face, or front crystal, of device 400. In some examples, FPC 1110and its embedded thermopile can be coupled between absolute temperaturesensor 442 and an inner surface of the device (at the same height withindevice 400 as PCB 440).

As described above in connection with thermopile 310 of FIG. 3 , FPC1110 can include a thermopile that includes multiple conductive paths ortraces. The thermopile of FPC 1110 can be formed across at least twolayers of FPC 1110, with a first layer for conductive paths of a firstconductive material/first metal, with a first Seebeck coefficient (e.g.,conductive paths 314), and with a second layer for conductive paths of asecond conductive material/second metal, with a second Seebeckcoefficient that is different from the first Seebeck coefficient (e.g.,conductive paths 316). The first metal can be copper (Cu), and thesecond metal can be a copper-nickel (CuNi) alloy/constantan. In someexamples, a conductive path of the first metal, formed in a first layerof FPC 1110, can partially overlap up to two conductive paths of thesecond metal, formed in the second layer of FPC 1110. Conductive pathsof different metals can be connected across different layers using vias.

Copper conductive paths can be coupled to constantan conductive paths,at a hot junction of FPC 1110 (e.g., the portion of FPC 1110 co-locatedwith absolute temperature sensor 442), and at a cold junction (e.g., theportion of FPC 1110 secured to back face 450). As an example, a firstcopper path in a first layer can extend from a cold junction of FPC 1110to its hot junction, and can be coupled to a first constantan path atthe hot junction (e.g., by a via, or another inter-layer connector). Thefirst copper path and the first constantan path can collectively form afirst thermocouple. The first constantan path can then extend from thehot junction of FPC 1110 to its cold junction, and can be coupled to asecond copper path at the cold junction (e.g., by a via, or anotherconnector). The second copper path can then extend from the coldjunction of FPC 1110 to its hot junction, and can be coupled to a secondconstantan path at the hot junction (e.g., by a via, or anotherconnector). The second copper path and the second constantan path cancollectively form a second thermocouple that can be coupled in serieswith the first thermocouple. In accordance with the arrangement of thisexample, the first constantan path can partially overlap one or more ofthe first and second copper paths used to form the thermopile embeddedwithin FPC 1110.

FIG. 12A illustrates a cross-sectional side view of an exemplaryflexible circuit with an inner layer used for signal propagation andouter ground layers that protect the inner layer according to someexamples of the disclosure. FPC 1200 can include a first outer layer1210 including copper flood or patterning (e.g., a shield layer, aground/fixed potential layer, etc.) and a second outer layer 1230including copper flood or patterning (e.g., a shield layer, aground/fixed potential layer, etc.). An inner layer 1220 can includecopper patterning for signal routing (e.g., signal paths). Connectionsbetween the layers 1210 and 1230 can be formed using a via 1240, or anyother suitable inter-layer connector available on FPC 1200. FPC 1200illustrates signal lines in layer 1220 to convey data signals, or anyother suitable type of signal from one end of FPC 1200 to the other(ends of FPC 1200 are not illustrated in FIG. 12A, which is across-sectional side view). In some examples, integrating a thermopileinto a FPC that already exists within device 400 can reduce designcomplexity and to conserve space within the device. However, as shown inFIG. 12A, all the layers of FPC 1200 can be copper layers, whereas athermopile requires layers corresponding to two different conductivemartials/metals, with different respective Seebeck coefficients.

FIG. 12B illustrates a cross-sectional side view of a modified FPC 1250with an integrated thermopile based on the exemplary flexible circuit ofFIG. 12A according to some examples of the disclosure. In some examples,one of the outer layers (e.g., outer layer 1210) in FIG. 12A can beformed of a different material/metal, such as constantan, orcopper-nickel (CuNi) alloy outer layer 1212. Though copper may have alower resistance (e.g., per unit area) than constantan, an outer layer1212 formed from constantan can be used to provide a ground plane, orshielding layer for the inner layer 1220.

The thermopile can be implemented in the thermopile routing region 1290adjacent to via 1240, in FPC 1250 (e.g., adjacent to existing patternedor flooded traces in layers 1210, 1220, and 1230. Thermopile routingregion 1290 can include conductive paths patterned from the constantanouter layer 1212 and the copper inner layer 1220. As an example, a firstcopper path within inner layer 1220 can extend from a cold junction ofFPC 1250 to its hot junction, and can be coupled to a first constantanpath within outer layer 1212 at the hot junction (e.g., by a via, oranother inter-layer connector). The first copper path and the firstconstantan path can collectively form a first thermocouple. The firstconstantan path within outer layer 1212 can then extend from the hotjunction of FPC 1250 to its cold junction, and can be coupled to asecond copper path within inner layer 1220 at the cold junction (e.g.,by a via, or another connector). The second copper path within innerlayer 1220 can then extends from the cold junction of FPC 1250 to itshot junction, and can be coupled to a second constantan path withinouter layer 1212 at the hot junction (e.g., by a via, or anotherconnector). The second copper path and the second constantan path cancollectively form a second thermocouple that can be coupled in serieswith the first thermocouple. In accordance with the arrangement of thisexample, the first constantan path within the outer layer 1212 canpartially overlap one or more of the first and second copper pathswithin the inner layer 1220 used to form the thermopile embedded withinFPC 1250.

The arrangement shown in FIG. 12B illustrates a three-layer FPC 1250 tomaintain flexibility of the FPC. However, it should be understood, thatin some examples, a fourth constantan layer can be added to the FPC ofFPC 1250, and a thermopile can be integrated with the FPC usingconductive paths patterned in the constantan layer and conductive pathspatterned in the layers 1210, 1220 and/or 1230.

FIG. 13A illustrates a cross-sectional side view of an exemplaryflexible circuit with a first segment using an inner layer for datasignal propagation and a second segment using an outer layer (or outerlayers) for power signal propagation according to some examples of thedisclosure. FPC 1300 can include first outer layer segments 1310-1 and1310-2 including copper flood or patterning (e.g., a shield layer/aground layer, a power propagation layer, etc.) and second outer layersegments 1330-1 and 1330-2 including copper flood or patterning (e.g., ashield layer/a ground layer, a power propagation layer, etc.). Innerlayer segments 1320-1 and 1320-2 can including copper patterning forsignal routing (e.g., signal paths). Though outer layer segments 1310-1and 1310-2 are formed on the same layer, they can be electricallyisolated or otherwise electrically separated as represented by a gapbetween segments 1310-1 and 1310-2. Similarly, outer layer segments1330-1 and 1330-2 can be formed on the same layer, but can beelectrically isolated or otherwise electrically separated as representedby a gap between segments segment 1330-1 and 1330-2). It should beunderstood however, that FPC 1300 can have a contiguous dielectric layerbetween the conductive layers shown.

Connections between the layers 1310-1, 1320-1, and/or 1330-1 can beformed using one or more vias (e.g., vias 1340), or any other suitableinter-layer connector available on FPC 1300, in a similar manner asdescribed for FPC 1200. FPC 1300 illustrates signal lines used to conveydata signals in layer 1320-1 and 1320-2 from one end of FPC 1300 to theother (ends of FPC 1300 are not illustrated in FIG. 13A, which is across-sectional side view). FPC 1300 also illustrates a power signallines in layer 1310-2 and/or 1330-2 from one end of FPC 1300 to theother (ends of FPC 1300 are not illustrated in FIG. 13A, which is across-sectional side view). In some examples, integrating a thermopileinto a FPC that already exists within device 400 can reduce designcomplexity and to conserve space within the device. However, as shown inFIG. 13A, all the layers of FPC 1300 can be copper layers, whereas athermopile requires layers corresponding to two different conductivematerials/metals, with different respective Seebeck coefficients.

FIG. 13B illustrates a cross-sectional side view of a modified FPC 1350with an integrated thermopile based on the exemplary flexible circuit ofFIG. 13A according to some examples of the disclosure. In some examples,because one or more of outer layers 1310-2 and 1330-2 can route powersignals, these layers cannot be replaced with constantan, orcopper-nickel (CuNi) alloy outer layer, without incurring a significantincrease in power dissipation caused by the higher resistance ofconstantan (relative to copper). Thus, in some examples, integration ofthe thermopile into the FPC can be achieved by replacing inner layer1320-1 and 1320-2 of copper with inner layer 1322 of constantan instead.

The thermopile can be implemented in the thermopile routing region 1390adjacent to via 1341, in FPC 1350 (e.g., adjacent to existing patternedor flooded traces in layers 1310-2, 1320-2, and 1330-2 of the secondconnector of FPC 1300. Thermopile routing region 1390 can includeconductive paths patterned from the constantan inner layer 1322 and thecopper outer layer 1310-1/1310-2. As an example, a first copper pathwithin the copper outer layer can extend from a cold junction of FPC1350 to its hot junction, and can be coupled to a first constantan pathwithin inner layer 1322 at the hot junction (e.g., by a via, or anotherinter-layer connector). The first copper path and the first constantanpath can collectively form a first thermocouple. The first constantanpath within inner layer 1322 can then extends from the hot junction ofFPC 1350 to its cold junction, and can be coupled to a second copperpath within the outer layer at the cold junction (e.g., by a via, oranother connector). The second copper path within the outer layer canthen extend from the cold junction of FPC 1350 to its hot junction, andcan be coupled to a second constantan path within inner layer 1322 atthe hot junction (e.g., by a via, or another connector). The secondcopper path and the second constantan path can collectively form asecond thermocouple that can be coupled in series with the firstthermocouple. In accordance with the arrangement of this example, thefirst constantan path within the inner layer 1322 can partially overlapone or more of the first and second copper paths within the outer copperlayer used to form the thermopile embedded within FPC 1350.

FIG. 14 illustrates an example process of estimating a temperature(e.g., using an absolute temperature sensor and a heat flux sensor)inside and outside a device according to some examples of thedisclosure. At 1402, the system (e.g., computing system 200) can measureabsolute temperature using an absolute temperature sensor (e.g.,absolute temperature sensor 254, 432, 442) located inside of a devicehousing of the system. As an example, an absolute temperature sensor 442can measure absolute temperature at a first location within device 400.Absolute temperature sensors be implemented using a negative temperaturecoefficient (NTC) temperature sensor, a resistance temperature detector(RTD), or a diode based temperature sensor. A temperature measurementfrom an absolute temperature sensor can correspond to a temperature at afirst location (e.g., corresponding to a location of the absolutetemperature sensor and/or to the local area around the sensor).

At 1404, the system can measure a temperature gradient. In someexamples, the temperature gradient can be between a first location(e.g., corresponding to the absolute temperature sensor) and second,different location within the device. In some examples, the systemmeasures the temperature gradient (temperature differential) using athermopile or other heat flux sensor 256, as described herein.

At 1406, the system can estimate absolute temperature at the second,different location based on the absolute temperature measured by anabsolute temperature sensor and the temperature gradient/differentialmeasured by a heat flux sensor 256 (e.g., a thermopile 310 as shown FIG.3 ). In some examples, the second location can be within the device, butseparated from the first location by the thermopile. In some examples,the system can calculate the sum of the absolute temperature measurementat the first location and the temperature gradient measurement toestimate an absolute temperature at a second location (e.g., at an endthe thermopile or other heat flux sensor). In some examples, such as theexample of FIG. 6A, a thermopile can be embedded within the rigid PCB ofbarrier 610 (and/or PCB 440). A first end of the thermopile/barrier canbe coupled to an underside of PCB 440, and a second end of thethermopile/barrier can be coupled to back face 450. In some suchexamples, the system (e.g., host processor 210 or temperature sensorcontroller 240) can estimate an absolute temperature at back face 450 byadding a temperature gradient/differential measured by the thermopileembedded within the rigid PCB of barrier 610, to an absolute temperaturemeasurement generated at absolute temperature sensor 442. In someexamples, a thermopile can be implemented using a flexible printedcircuit 1110 having a first end coupled to absolute temperature sensor442/PCB 440, and a second end coupled to back face 450. In some suchexamples, the system (e.g., host processor 210 or temperature sensorcontroller 240) can estimate an absolute temperature at back face 450 byadding a temperature gradient/differential measured by the thermopileembedded within FPC 610, to an absolute temperature measurementgenerated at absolute temperature sensor 442.

In some examples, such as the example of FIG. 4B, multiple thermopilescan be used with one or more absolute temperature sensors to estimatetemperatures at multiple locations within the device in a similarmanner.

At 1408, the system can optionally estimate a component temperatureassociated with a heat flux sensor 256. As an example, when heat fluxsensor 256 is a thermopile with an end coupled to a location at or neara component of the system, the system (e.g., temperature sensorcontroller 240, host processor 210) can estimate the temperature of theparticular component based on the estimated absolute temperature at theend of the thermopile (e.g., the absolute temperature estimated at1406). In some examples, a device can be characterized to estimate atemperature of a component based on the estimated temperature at thesecond location. Based on the component temperature estimated at 1408,the system (e.g., power management circuitry 209) can makedeterminations about its operating conditions, such as determiningwhether any particular component temperature exceeds a predefined uppertemperature boundary associated with impaired or unsafe componentoperation. In some examples, the system can report componenttemperatures, or conditions determined to be associated with componenttemperatures, to a user. In some examples, the components of the systemcan include one or more processors, wireless communication circuitry,global positioning system, optical emitters, etc.

Additionally or alternatively, at 1410, the system can compensate anoptical sensor or compensate an estimate of one or more physiologicalsignals based on the absolute temperature estimated at 1406. As anexample, light emitters 204 can optionally include LEDs that can producea very narrow band of visible or non-visible light, with an associatedcentroid wavelength of that band. The centroid wavelength of an LED canchange, or drift based on a temperature of the LED. When the absolutetemperature estimated at 1406 corresponds to a temperature associatedwith a particular light emitter 204, a compensation model (not shown)can be used to determine the centroid wavelength of the particular lightemitter, which can be provided as a parameter to the compensation model.In some examples, the compensation can change the stimulation applied toan emitter so that the light emitted falls within the desired narrowband of light. In some examples, the estimated wavelength can beprovided to processing circuitry used to estimate physiological signalsbased on sensor data from the light detector 206 associated with theparticular light emitter 204 to compensate for the effects of thethermal drift of the emitter on the estimate. It should be noted thatthe wavelength estimation techniques disclosed herein can be applied toany LED/PD (e.g., light emitting diode/photodetector or photodiodecomponents that are located at any location in the device.

Additionally or alternatively, at 1412, the system can estimate ambientair temperature outside the device (e.g., outside housing 410) and/orestimate a surface temperature of the device, such as a temperature onthe front crystal or front face of the device. In some examples, theabsolute temperature at the second location estimated at 1406corresponds to a temperature at front face 162 of a device 160, as shownin FIG. 1F. In other examples, the absolute temperature estimated at1406 corresponds to a temperature at the top surface of PCB 420, locatedat the front face of housing 410 of FIG. 4A. A heat flux model, such asmodel 500 or 550 illustrated in FIGS. 5A and 5B, can be used inconjunction with the temperature estimated at 1406 (corresponding to T₂in model 500, and T_(FC) in model 550), to estimate an ambient airtemperature outside of the device (T_(A), or T_(AMBIENT)), sometimesreferred to as an environmental temperature. In some examples, theestimated ambient air temperature outside of the device can be reportedto the user (e.g., displayed on a display).

Additionally or alternatively, at 1414, the system can estimate skintemperature, such as the temperature of skin at a user's wrist,forehead, temples, or any other body surface that contacts a housing ofthe system. In some examples, the absolute temperature estimated at 1406corresponds to a location within the housing that contacts the user'sbody at its outer surface. As an example, the absolute temperatureestimated at 1406 can correspond to back face 450, which can have anouter surface that contacts a wrist of user 460 (as shown in FIG. 4A).As another example, the absolute temperature estimated at 1406 cancorrespond to a front face of device 400 (e.g., adjacent to PCB 420),which can have an outer surface that can be pressed against the foreheadof user 460 or another person (e.g., by moving the device so that itsfront face is pressed against the forehead). In some examples,temperature sensor controller 240 uses the temperature estimated at 1406as an input to a heat flux model, such as model 500 or 500 illustratedin FIGS. 5A and 5B, to estimate skin temperature associated with user460 (T_(WRIST), T_(S), or T_(C) corresponding to corrected skintemperature). In some examples, the estimated skin temperatureassociated with user 460 can be tracked and/or reported to the user(e.g., displayed on a display).

Additionally or alternatively, at 1416, the system can estimate corebody associated with user 460. In some examples, the absolutetemperature estimated at 1406 can correspond to wrist temperaturemeasured at a back face of the device, and can be used to estimate bodytemperature when the user is in a vasodilation condition (e.g., at nightwhen a user sleeps). In other examples, the absolute temperatureestimated at 1406 can correspond to forehead or temple temperature, andcan be used to measure temperature at a different region of the body ator closer to core body temperature using a front face, or strap of thedevice to enable measurements at the forehead or temples, even duringvasoconstriction. For example, a user may bring the front face of awearable device into contact with the forehead to estimate core bodytemperature, or secure a strap of the wearable device around the head toestimate core body temperature. In some examples, the estimated corebody temperature can be tracked and/or reported to the user (e.g.,displayed on the display). Generally, at 1416, the system can estimatecore body temperature associated with a user, based on skintemperatures, such as those estimated at 1414. Accordingly, 1416 canoptionally be performed using the estimated skin temperature from 1414as a parameter input to a heat flux model (e.g., models 500 or 550 fromFIGS. 5A and 5B), to estimate a user's core body temperature.

FIG. 15 illustrates an example process of operating a device fortemperature sensing operations according to some examples of thedisclosure. For example, process 1500 can determine that an electronicdevice used for temperature sensing operations correspond to conditionssuitable for a qualifying temperature sensing measurement. For example,the conditions for a valid temperature sensing measurement can includesufficient contact between an outer surface of the device and a user'sskin and/or an absence of motion (e.g., a stationary user withrelatively stationary contact between a front face of a device and auser's forehead). At 1502, the system (e.g., host processor 210) canreceive a user request to initiate temperature measurement at an outersurface of the device. In some examples, in response to the userrequest, the system (e.g., processor 210) can determine whether one ormore criteria associated with a contact condition of the device at itsouter surface are satisfied. As an example, when the user requeststemperature measurement at the wrist (based on temperature measurementat the back face), the system can determine whether criteria associatedwith a contact condition between the back face and the user's wrist havebeen satisfied. As another example, when the user requests temperaturemeasurement at the forehead or temples (based on temperature measurementat the front face of a device or at the strap), the system can determinewhether criteria associated with a contact condition between the frontface and the user's forehead or between the strap and the user's templeshave been satisfied. As yet another example, when the user requeststemperature measurement of the ambient air surrounding the device, thesystem can determine whether criteria associated with a contactcondition between the front face and the ambient air (e.g., the absenceof a contact condition between the front face and any object) have beensatisfied. It should be understood that the criteria may be differentdepending on the type of temperature measurement. For example, contactmay be desirable with the skin for a body temperature measurement, butmay be undesirable for an ambient air temperature measurement. In someexamples, process 1500 can be performed without requiring a user inputto request a temperature sensing measurement (e.g., an opportunisticmeasurement). In some examples, when a user input to request atemperature measurement is used a subset of the sensors and/or relaxedassociated criteria can be used to determine the contact condition ofthe device outer surface is suitable for the temperature measurement.

At 1504, the system (e.g., touch and display controller 216) monitorstouch sensors such as touch screen 220, to determine whether sensor datafrom touch screen 220 indicate that a contact condition of the devicecorresponding to the requested temperature measurement has beensatisfied. As an example, when a user requests forehead temperaturemeasurement at 1502, touch screen 220 can be monitored for dataindicative of a user's forehead or temples contacting the touch screen220. In some examples, the one or more criteria can include a criterionthat is satisfied when an object is detected in contact with the touchsensor. In some examples, the one or more criteria include a criterionthat is satisfied when an object greater than threshold area of thetouch sensor. In some examples, the one or more criteria include acriterion that the object corresponds to human tissue (e.g., excludingfloating objects such as water droplets). As another example, when auser requests ambient air temperature measurement, touch screen 220 canbe monitored for data indicative of no objects being in contact with thetouch screen 220 (e.g., to ensure the front face and touch screencompletely contact the ambient air outside the device).

Additionally or alternatively, at 1506, the system (e.g., host processor210, temperature sensor controller 240) can monitor motion and/ororientation sensors 230. In some examples, the system can use the datafrom motion and/or orientation sensors 230 to determine that a user isstationary and/or that a contact between the user and the outer surfaceof the device is stationary (e.g., less than a threshold amount ofmovement by the user and/or by the device relative to the user). In someexamples, motion of the user and/or motion of the device relative to theuser's skin can introduce noise (motion artifacts) into the temperaturemeasurement. In some examples, the system can use the data from motionand/or orientation sensors 230 to determine whether a user has performeda gesture to satisfy the contact condition of the requested temperaturemeasurement. The gesture to satisfy the contact condition can includemechanical gestures that indicate a device has been moved to anappropriate location for temperature measurement/estimation. As anexample, when a user requests forehead temperature measurement at 1502,motion and/or orientation sensors 230 can be monitored for dataindicative of a gesture to bring a wearable device from a first heightto a second height associated with the user's forehead or temples and/orto rotate the wrist to bring the front face of the device to theforehead.

Additionally or alternatively, at 1508, the system (e.g., temperaturesensor controller 240, power dissipation monitoring circuitry 213, etc.)can monitor temperature sensors 250 and/or power dissipation sensors. Insome examples, the monitoring can determine whether sensor data fromsensors 250 indicate that a user has positioned the device such that itcontacts a surface corresponding to the requested temperaturemeasurement. As an example, sensor data from temperature sensors 250 canbe monitored for temperatures within an expected range of temperaturesfor the requested temperature measurement (e.g., optionally a firstexpected temperature range for wrist temperature measurement, a secondexpected temperature range for forehead temperature measurement, a thirdexpected temperature range for ambient air temperature, etc.). In someexamples, the one or more criteria can include a criterion that issatisfied when the measured temperature is within an expected range. Insome examples, the monitoring can determine the power usage by thedevice (or a subset of components) and the associated temperatureeffects on the system or directly measure the temperature of the system.In some examples, when the power usage or the temperature is too high,the system can forgo a temperature measurement or take other action toimprove conditions or to suppress the measurement from being reported.In some examples, the one or more criteria can include a criterion thatis satisfied when the power usage and/or the internal device temperatureare below a power usage and/or temperature threshold.

At 1510, the system can determine whether the outer surface of thedevice has satisfied one or more criteria associated with a contactcondition for the requested temperature measurement. When the one ormore criteria are satisfied, the system can estimate the temperature.When the one or more criteria are not satisfied, the system can forgoestimating the temperature (or the system can attempt to improveconditions, compensate the temperature estimate, and/or suppress theestimate from being reported to the user). The system can estimate thetemperature using temperature sensors 250 including a thermopile asdescribed herein (and optionally heat flux models as described herein).

FIG. 16 illustrates another example process of operating a device fortemperature sensing operations according to some examples of thedisclosure. For example, process 1600 can estimate temperature whencertain criteria are satisfied (e.g., the criteria relating to thoseconditions describe with reference to process 1500 and/or thetemperature and/or power consumption conditions at the device, asdescribed herein). At 1602, the system can receive a user request toinitiate temperature measurement at an outer surface of the device(e.g., wrist, forehead, etc.).

At 1604, the system can determine whether device conditions permitaccurate temperature measurement. In some examples, the system canmonitor sensors such as touch screen 220, motion and/or orientationsensors 230, optical sensors 211, temperature sensors 250, powerdissipation monitoring circuitry 213, etc. In particular, sensor datafrom these sensors can be monitored to determine whether one or morecriteria are satisfied for a temperature measurement. The one or morecriteria can include contact conditions between the device and thetarget of temperature measurement (e.g., ensuring a user and the deviceis relatively stationary, ensuring good contact between the device and auser's skin, wrist, forehead, temples, or ambient air) and/or theaggressor conditions of the device (e.g., internal device temperatureand/or power consumption are above a threshold, thermal aggressiveprocesses are in progress (e.g., GPS tracking, etc.).

When, the system determines that conditions do not permit accuratetemperature measurement, the system can forgo temperature measurement oralternatively attempt to improve device conditions. When, the systemdetermines that device conditions permit accurate temperaturemeasurement, the system can estimate temperature as described herein(e.g., with respect to process 1400). In some examples, the system cantrack and/or report (e.g., display on the display) the estimatedtemperature to the user.

In some examples, at 1606, the system can implement measures to improvedevice conditions. In some examples, the system can provide a user withinstructions to meet the one or more criteria. For example, the user canbe instructed (e.g., via audio, visual, textual cues) to reduce motionand/or improve contact between the outer surface of the device and theuser's skin. In some examples, the system (e.g., host processor 210,power management circuitry 209) can suppress or interrupt power deliveryto components associated with high heat dissipation, or thermalaggression, especially those components located near temperature sensors250. As another example, host processor 210 can ask the user tothrottle, terminate, or delay system processes that may be thermallyaggressive. For example, the system can reduce display intensity,power-down a high power processor, temporarily disable or throttlecommunications using cellular, Wi-Fi, Bluetooth, or GPS. The system canreturn to determine whether conditions are met for accurate and reliabletemperature sensor measurements at 1604 as or after the systemimplements the measures to improve device conditions.

FIG. 17 illustrates another example process of operating a device fortemperature sensing operations according to some examples of thedisclosure. For examples, process 1700 can cause the device to report,store and/or display temperature estimates when one or more criteria aresatisfied (e.g., indicative of a quality temperature measurement),whereas the process 1700 can cause the device to suppress or discard(e.g., not report, store and/or display) a temperature estimate when theone or more criteria are not satisfied. At 1702, that system can receivea user request to initiate temperature measurement as described hereinsimilarly at 1502 and 1602 in processes 1500 and 1600. At 1704, thesystem can (host processor 210 and/or temperature sensor controller 240,etc.) can estimate temperature at or outside an outer surface of thedevice (e.g., as described with reference to FIG. 14 or FIG. 16 and notrepeated here for brevity).

At 1706, the system can determine whether device conditions permitaccurate temperature measurement (e.g., in the same or in a similarmanner as described with reference to 1510 of process 1500 or 1604 ofprocess 1600, and not repeated here for brevity).

When the system determines that device conditions during the measurementdo not permit accurate temperature measurement (e.g., because ofmotion/poor contact or because of power/temperature conditions on thedevice), the system can discard the temperature estimate and/or foregoreporting the temperature estimated at 1708. Alternatively, usingmeasurements from the power dissipation monitoring circuitry and/or amodel to derive the impact of the power consumption of certaincomponents (e.g., thermal aggressors of system 200), the temperatureestimate can be compensated to remove the impact of the thermalaggressors.

When the system determines that device conditions did permit accuratetemperature measurement, the system can store and/or report (1710) theestimated temperature to a user (e.g., display on the display).

As described herein, in some examples, the system includes two or moreabsolute temperature sensors to estimate temperature inside or outsidethe device. FIG. 18 illustrates a cross-sectional side view of anexemplary electronic device including one or more printed circuit boardsand temperature sensing circuitry according to some examples of thedisclosure. Wearable device 1800 can correspond to the wearable device400 of FIG. 4A, with additional features. Also, wearable device 1800 cancorrespond to a device 150 of FIG. 1E and/or 160 of FIG. 1F (or moregenerally can correspond to any of the electronic devices illustrated byFIGS. 1A-1G). Further, one or more like elements and like features ofwearable device 1800 optionally correspond to one or more like elementsand like features of wearable device 400 of FIG. 4A, wearable device 600of FIG. 6A, and/or wearable device 1100 of FIG. 11A. Device 1800 caninclude a housing 1810 secured to user 1860 via a strap 1812 or anyother suitable fastener (e.g., corresponding to strap 154 and housing164). In some examples, device 1800 can be secured to the user 1860(e.g., exposed skin on the user's body). Device 1800 can correspond to awatch, a fitness tracker, bracelet, wrist band, or any other device(e.g., optionally used to measure physiological signals associated withuser 1860). Device 1800 can attach to user 1860 around the wrist, arm,head, neck, or on any exposed surface of the body that is suitable formeasuring physiological signals associated with the user.

Multiple printed circuit boards (PCBs) 1820, 1830, and 1840 areillustrated inside housing 1810. For example, PCB 1820 can be locatedinside device 1800, at a front face (sometimes referred to as a “frontcrystal”). In some examples, PCB 1820 can be used to implement a touchsensor panel, display and/or touch screen (e.g., touch screen 220)disposed below the front face. In the illustrated example, an absolutetemperature sensor 1821 (e.g., a discrete absolute temperature sensor)is mounted to the PCB 1820. In some examples, the absolute temperaturesensor 1821 is mounted on a first side (e.g., bottom side) of the PCB1820. In some examples, the absolute temperature sensor 1821 is mountedon a second side (e.g., top side) of the PCB 1820. In some examples, theabsolute temperature sensor 1821 is mounted on two sides of the PCB 1820(e.g., a top side and a bottom side) or is embedded within the PCB 1820.

PCB 1830 can be located inside device 1800, between PCB 1820 and 1840.In some examples, PCB 1830 includes host processor 210, program storage202, touch and display controller 216, optical sensor controller 212and/or temperature sensor controller 240. In the illustrated example, anabsolute temperature sensor 1832 (e.g., similar to absolute temperaturesensor 254 of FIG. 2 ) is mounted to PCB 1830. Although illustrated asmounted to the top side of PCB 1830, absolute temperature sensor 1832 isoptionally embedded within the PCB 1830 or mounted to the bottom side orboth the top and bottom sides of PCB 1830.

In the illustrated example, PCB 1840 is located inside device 1800,below PCB 1830 at or in proximity to a back face 1850 (sometimesreferred to as a “back crystal”). In some examples, PCB 1840 canadditionally or alternatively include an absolute temperature sensor1842 (e.g., a discrete absolute temperature sensor). Althoughillustrated as mounted to the top side of PCB 1840, absolute temperaturesensor 1842 is optionally embedded within the PCB 1840, mounted to thebottom side or both the top and bottom sides of PCB 1840. It should benoted that the absolute temperature sensors discussed with reference toFIG. 18 are optionally discrete absolute temperature sensors and areoptionally manufactured by the same entity or different entities.

In some examples, absolute temperature sensor 1842 can be separated fromback face 1850 by PCB 1840, and PCB 1840 can be separated from housing1810 (e.g., without direct contact with housing 1810 due to theexistence of one or more intervening layers or an air gap). In theillustrated example, the device 1800 includes conductive segments 1852(e.g., rods, vias, or another type of conductive segments, includingcopper or another type of thermally conductive material). Each of theillustrated conductive segments 1852 optionally represent one or moreconductive segments, though in various examples, more or fewerconductive segments are included in the electronic device. Theconductive segments optionally reduce thermal resistance (increasethermal conductivity) between PCB 1830 and back crystal 1850.

PCB 1840 can include optical sensors 211 configured to emit light anddetect light through back face 1850 (e.g., light emitters and detectorsmounted on the opposite side of PCB 1840). The number of PCBs, thenumber of temperature sensors, and placement of PCBs and distribution ofcomponents between the PCB s shown in FIG. 18 is representative andnon-limiting. For example, fewer or more PCBs can be used, fewertemperature sensors can be used (e.g., omitting either absolutetemperature sensor 1832 or absolute temperature sensor 1842), moretemperature sensors can be used than in the illustrated exemplarydevice, or the components of system 200 can be distributed differentlyacross the one or more PCBs. For examples, in some examples, the device1800 includes the absolute temperature sensors 1821 and 1842, withoutincluding the absolute temperature sensor 1832. Although the absolutetemperature sensors shown in FIG. 18 are mounted to a corresponding PCB,it is understood that temperature sensors are not limited to beingmounted to a PCB. One or more absolute temperature sensors can beintegrated into the device (e.g., bonded to the housing, integratedwithin another component or within a PCB, etc.). Additionally, more thanone absolute temperature sensor can be mounted to the same PCB.

In some examples, heat flux through device 1800 can be calculated usingone or more of discrete absolute temperature sensors 1821, 1832, 1842(with or without the use of a thermopile). As an example, thetemperature measured by sensor 1842 can be subtracted from thetemperature measured by sensor 1832 to determine a temperaturedifference between the absolute temperature sensors mounted to PCB 1830and PCB 1840. This temperature difference can then be used to calculateheat flux through the device, as well as estimating a temperatureoutside of the device (e.g., ambient air temperature at the back crystalor body temperature at back crystal 450).

FIG. 19 illustrates simplified schematic view of heat flux models for anelectronic device relative to a user's body according to some examplesof the disclosure. One or more like elements and like features of model1900 wearable device 1800 optionally correspond to like one or moreelements and like features of model 500 and/or model 550. In theillustrated example, a thermal resistance R_(FC) is in between a glassportion and a FC MLB (front crystal main logical board). In theillustrated example, an absolute temperature sensor that measurestemperature T₂ is disposed on the FC MLB (e.g., on the PCB 1820 of FIG.18 ). In the illustrated example, R₁₋₂ corresponds to a thermalresistance in between a distance between the absolute temperature sensorthat measures temperature T₂ and the absolute temperature sensor thatmeasures temperature T₁, which in the illustrated example, is belowsystem-in-package (SiP) circuitry (e.g., below the PCB 1830 of FIG. 18 )and mounted to a main logic board (e.g., the PCB 1840 of FIG. 18 ).

In addition, the model 1900 includes conductive segments for increasinga thermal conductivity (reducing a thermal resistivity R_(BC)) between aprinted circuit board (e.g., PCB 1840 and the back crystal (e.g., backcrystal 1850). In some examples, the electronic device includes amaterial (e.g., a substrate or an injected molded plastic material suchas a liquid crystal polymer) having a low thermal conductivity (and thusa high resistivity) in between the MLB (e.g., a logic board such as amain logic board) and the BC (back crystal). Such material may reduceundesirable coupling between components of the electronic device such asbetween the PCB and the back crystal. However, it is desirable to havean ample level of conductivity between the MLB and BC so that R_(BC)(e.g., R_(1-BC) of FIG. 5B) is reduced, which may likewise reduce a₀described above in the present disclosure with reference to model 550.Reducing error in temperature T_(BC) (e.g., a temperature correspondingthe back crystal that optionally contacts the user) may increase thermalcoupling to the skin of the user. To increase thermal conductivity(e.g., reduce R_(BC) (e.g., R_(1-BC) of FIG. 5B)), conductive segmentsare disposed between a printed circuit board (e.g., PCB 1840 of FIG. 18and the back crystal (e.g., back crystal 1850 of FIG. 18 ). In someexamples, a method for manufacturing the electronic device includesembedding and/or sprinkling conductive segments (e.g., vias such ascopper vias) in the material (e.g., an injected molded plastic materialsuch as a liquid crystal polymer) having a low thermal conductivity (andthus a high resistivity) in between the MLB and the BC. In someexamples, the method includes adding a thermal conductive pressuresensitive adhesive (PSA) to adhere to the layers in which the conductivesegments are added. In some examples, a method for manufacturing theelectronic device includes adding the conductive segments using aprocess like a laser direct structuring (LDS) process. Another benefitof increasing thermal conductivity between the BC (back crystal) and theMLB that is under the SiP circuitry is that a settling time for internalthermal aggressors may be reduced, due to the coupling of the heat fromthe internal thermal aggressors (e.g., a power component, a hapticengine, one or more processors, a system-in-chip circuitry, or anotherinternal thermal aggressor of the electronic device) to the skin of theuser through the conductive segments (e.g., the skin of the user canserve as a heat sink for the heat generated by the internal thermalaggressors of the electronic device). For example, thermal aggressornoise amplitude, a rise time, and a fall time may be reduced viaintegration of the conductive segments into the electronic device.

Therefore, according to the above, some examples of the disclosure aredirected to an electronic device (e.g., exemplary the electronic deviceof FIG. 18 ) that includes a plurality of absolute temperature sensorsincluding a first absolute temperature sensor (e.g., absolutetemperature sensor 1842 of FIG. 18 ) for measuring a first temperatureat a first location in the electronic device and a second absolutetemperature sensor (e.g., absolute temperature sensor 1821 of FIG. 18 )for measuring a second temperature at a second location in theelectronic device. The electronic device optionally also includes one ormore processors (e.g., host processor 210 of FIG. 2 and/or othercontrollers or processors of FIG. 2 ) for estimating a temperature of auser of the electronic device, using the first temperature measured bythe first absolute temperature sensor (e.g., T₁ as discussed withreference to FIG. 19 ), the second temperature measured by the secondabsolute temperature sensor (e.g., T₂ as discussed with reference toFIG. 19 ), a first thermal resistance between the first location in theelectronic device the second location in the electronic device (e.g.,R₁₋₂ as discussed with reference to FIG. 19 ) and a second thermalresistance between the electronic device and the user of the electronicdevice (e.g., R_(CONT) and/or R_(BC) as discussed with reference to FIG.19 ).

In some examples, the electronic device further includes a first printedcircuit board (e.g., PCB 1820) and a second printed circuit board (e.g.,PCB 1840). The first absolute temperature sensor (e.g., absolutetemperature sensor 1842) is optionally mounted to the first printedcircuit board and the second absolute temperature sensor (e.g., absolutetemperature sensor 1821) is optionally mounted to the second printedcircuit board. In some examples, the first absolute temperature sensor(e.g., absolute temperature sensor 1842) mounted to the first printedcircuit board and the second absolute temperature sensor (e.g., absolutetemperature sensor 1821) mounted to the second printed circuit board arevertically aligned in the electronic device (or within a thresholdhorizontal tolerance from being vertically aligned). In other examples,the first absolute temperature sensor and the second absolutetemperature sensor are not vertically aligned. In some examples, adistance between the first absolute temperature sensor and the secondabsolute temperature sensor in the electronic device is maximized. Inother examples, the distance is minimized. In yet other examples, thedistance is in between a maximum distance and a minimum allowabledistance within the electronic device (e.g., the distance is 3 mm, 5 mm,6 mm, or another distance). In some examples, the second printed circuitboard (e.g., PCB 1820) includes circuitry for a touch operation or adisplay operation of the electronic device and the first printed circuitboard (e.g., PCB 1820) includes circuitry for a physiological sensingoperation of a physiological attribute of the user of the electronicdevice.

In some examples, the electronic device further includes a third printedcircuit board (e.g., PCB 1830) between the first printed circuit boardand the second printed circuit board. In some examples,system-in-package (SiP) circuitry is mounted to the third printedcircuit board and the one or more processors are included in the SiPcircuitry. In some examples, the electronic device includes a thirdabsolute temperature sensor (e.g., absolute temperature sensor 1832)that is mounted to the third circuit board or anchored to a housing(e.g., housing 1810) of the electronic device.

In some examples, the second thermal resistance between the electronicdevice and the user of the electronic device corresponds to a thermalresistance of a contact interface between the electronic device and askin of the user of the electronic device (e.g., R_(CONT), such asdescribed with reference to the models 550 and 1900) and a thermalresistance of tissue of the user of the electronic device (e.g.,R_(PHYS), such as described and/or illustrated with reference to themodels 550 and/or 1900).

In some examples, the plurality of absolute temperature sensors of theelectronic device further includes a third absolute temperature sensorfor measuring a third temperature at a third location in the electronicdevice (e.g., absolute temperature sensor 1832) and a fourth absolutetemperature sensor configured to measure a fourth temperature at afourth location in the electronic device. In some examples, estimatingthe temperature of the user of the electronic device further uses thethird temperature measured by the third absolute temperature sensor andthe fourth temperature measured by the fourth absolute temperaturesensor. As such, in some examples, the corrected skin temperature of theuser, or T_(C), (such as described and/or illustrated with reference tothe models 550 and/or 1900) is optionally a function of the firsttemperature measure by the first absolute temperature sensor, the secondtemperature measured by the second absolute temperature sensor, thethird temperature measured by the third absolute temperature sensor, andthe fourth temperature measured by the fourth absolute temperaturesensor and thermal resistance values that are based on the respectivepositions of the four absolute temperature sensors relative to theelectronic device. As such, thermal resistances values different fromthe first thermal resistance and the second thermal resistance areoptionally used and are based at least on respective positions of thefour absolute temperature sensors relative to the electronic device. Insome examples, the electronic device includes more than four absolutetemperature sensors.

In some examples, the electronic device includes one or more conductivesegments (e.g., rods, vias, or another shape of conductive segments)extending from the first printed circuit board toward an externalsurface of a housing of the electronic device and configured to increasea thermal conductivity (or reduce the thermal resistance) between thesecond absolute temperature sensor and the external surface of thehousing, such as the conductive segments described with reference toFIGS. 18 and 19 . In some examples, the one or more conductive rods areconfigured to contact the housing, pass at least through a portion ofthe housing, or are exposed on an outer surface of the housing.

In some examples, the first thermal resistance between the firstlocation and the second location (such as R₁₋₂ described with referenceto the models 550 and 1900) is determined using empirical measurementsof one or more devices similar to the electronic device, withoutincluding the electronic device, such as via a factory calibrationprocess or testing process. For example, empirical measurements can bemade using a particular electronic device model and/or for a group oftest users. One or more of the empirically derived thermal resistancescan be used for temperature sensing by the electronic device of the samemodel type without requiring factory calibration of the thermalresistances for the electronic device.

In some examples, the first thermal resistance between the firstlocation and the second location (such as R₁₋₂ described with referenceto the models 550 and 1900) is determined using empirical measurementsof one or more devices, including the electronic device, such as via afactory calibration process or testing process.

In some examples, the second thermal resistance corresponding to thesecond thermal resistance (such as R_(CONT) and/or R_(PHYS) describedwith reference to the models 550 and 1900) between the electronic deviceand the user of the electronic device is determined using empiricalmeasurements that do not include measurement(s) of the user of theelectronic device.

In some examples, the first thermal resistance (such as R₁₋₂ describedwith reference to the models 550 and 1900) and/or the second thermalresistance (such as R_(CONT) and/or R_(PHYS) described with reference tothe models 550 and 1900) are determined using one or more stock keepingunits (SKUs) (e.g., product identifier) of the electronic device orparts of the electronic device. For example, the one or more processorscan perform operations for accessing the information corresponding tothe one or more SKUs of the electronic device and determining the firstthermal resistance (e.g., R₁₋₂ as discussed with reference to FIG. 19 )based on the one or more SKUs. The information corresponding to the oneor more SKUs optionally depend on a size of the electronic device, amaterial of a housing of the electronic device, a producer of theelectronic device and/or parts thereof, and/or other elements of theelectronic device and/or parts of thereof. For example, a first exampleof the device optionally includes a first set of SKU(s) indicating afirst set of geometrical dimension(s) and material(s) of the device anda second example of the device optionally includes a second set ofSKU(s) indicating a second set of geometrical dimensions and/ormaterials of the device, optionally different from the first set ofgeometrical dimensions and/or materials of the device. The thermalresistances derived for the first example of the device are optionallydifferent from the thermal resistances derived for the second example ofthe device, based at least in part of the first set of SKU(s) and thesecond set of SKU(s), respectively. In some examples, the one or morethermal resistances can be stored or programmed in registers or othermemory circuitry based on the SKU, and these one or more thermalresistances can be accessed for use in temperature sensing describedherein.

In some examples, the second thermal resistance (such as R_(CONT) and/orR_(PHYS) described with reference to the models 550 and 1900) isdetermined during calibration (e.g., by the user and/or fieldcalibration) while contacting the user of the electronic device (e.g.,while worn by the user).

In some examples, the electronic device can operate in at least twopower modes, including a first power mode and a second power mode,whereby power usage of the electronic device in the second power mode(e.g., which optionally includes operation of the lower power processor211-1 of FIG. 2 ) is less than power usage of the electronic device inthe first power mode (which optionally includes operation of the higherpower processor 211-2 of FIG. 2 ). In some examples, the one or moreprocessors estimate the temperature of the user while the electronicdevice is operating in the second power mode. In some examples, the oneor more processors estimate the temperature of the user while theelectronic device is operating in the first power mode. In someexamples, the one or more processors can estimate the temperature of theuser while the electronic device is operating in the first power modeand estimate the temperature of the user while the electronic device isoperating in the second power mode. In some examples, the use oftemperature sensing in different modes is optionally determined based onthe suitability of conditions for a qualifying temperature sensingmeasurement as described with reference to process 1500.

In some examples, a first distance between the second location (e.g.,the location at which the absolute temperature sensor 1821 of FIG. 18measures the second temperature) and the user of the electronic device(e.g., skin of the user 1860) is greater than a threshold distance suchas 2 mm, 3 mm, 5 mm, 6 mm, or another threshold distance. In someexamples, a second distance (e.g., the distance 1862 of FIG. 18 )between the second location (e.g., the location at which the absolutetemperature sensor 1821 of FIG. 18 measures the second temperature) andthe back crystal of the electronic device (e.g., the back face 1850) orsurface of the electronic device configured to contact skin of the useris greater than a threshold distance such as 2 mm, 3 mm, 5 mm, oranother threshold distance that is optionally greater than 5 mm. In someexamples, a third distance between the first location (e.g., thelocation at which the absolute temperature sensor 1842 of FIG. 18measures the second temperature) and the user of the electronic device(e.g., skin of the user 1860) is greater than a threshold distance suchas 1 mm, 2 mm, 5 mm, or another threshold distance.

In some examples, a method is performed at an electronic device (e.g.,exemplary the electronic device of FIG. 18 ) including a plurality ofabsolute temperature sensors including a first absolute temperaturesensor (e.g., absolute temperature sensor 1842 of FIG. 18 ) and a secondabsolute temperature sensor (e.g., absolute temperature sensor 1821 ofFIG. 18 ). The method includes measuring a first temperature at a firstlocation in the electronic device using the first absolute temperaturesensor (e.g., T₁ as discussed with reference to FIG. 19 ), measuring asecond temperature at a second location in the electronic device,different from the first location, using the second absolute temperaturesensor (e.g., T₂ as discussed with reference to FIG. 19 ), andestimating a temperature of a user of the electronic device, using: thefirst temperature measured by the first absolute temperature sensor(e.g., T₁ as discussed with reference to FIG. 19 ), the secondtemperature measured by the second absolute temperature sensor (e.g., T₂as discussed with reference to FIG. 19 ), a first thermal resistancebetween the first location in the electronic device and the secondlocation in the electronic device (e.g., R₁₋₂ as discussed withreference to FIG. 19 ), and a second thermal resistance between theelectronic device and the user of the electronic device (e.g., R_(CONT)and/or R_(BC) as discussed with reference to FIG. 19 ). In someexamples, the method includes one or more additionally operationsdescribed in this present disclosure.

In some examples, a non-transitory computer readable storage mediumstoring one or more programs, the one or more programs comprisinginstructions, which when executed by one or more processors of anelectronic device (e.g., exemplary the electronic device of FIG. 18 )including a plurality of absolute temperature sensors including a firstabsolute temperature sensor (e.g., absolute temperature sensor 1842 ofFIG. 18 ) and a second absolute temperature sensor (e.g., absolutetemperature sensor 1821 of FIG. 18 ), cause the electronic device toperform the method discussed above. Additionally, in some examples, themethod includes one or more additional operations described in thedisclosure.

Some examples of the disclosure are directed to a heat flux sensorcomprising: a printed circuit board (PCB) comprising a thermopile,wherein the thermopile comprises a plurality of thermocouples in series,and wherein the PCB is rigid; a plurality of vias in the PCB comprisingone or more first vias from a first layer of the PCB to a second layerof the PCB comprising a first conductive material with a first Seebeckcoefficient and one or more second vias from the first layer of the PCBto the second layer of the PCB comprising a second conductive materialwith a second Seebeck coefficient, different from the first Seebeckcoefficient; a plurality of conductive traces on the first layer of thePCB and on the second layer of the PCB, the plurality of conductivetraces interconnecting the plurality of vias; and sensing circuitrycoupled to the thermopile and configured to measure a voltageproportional to a temperature gradient between the first layer and thesecond layer of the PCB. Additionally or alternatively, in someexamples, each of the plurality of thermocouples comprises one of thefirst vias and one of the second vias coupled by one of the plurality ofconductive traces. Additionally or alternatively, in some examples, theplurality of vias comprise through-hole vias from the first layer to thesecond layer. Additionally or alternatively, in some examples, the PCBcomprises a third layer and a fourth layer, the third layer and thefourth layer between the first layer and the second layer; and theplurality of vias comprise through-hole vias. Additionally oralternatively, in some examples, the PCB comprises a third layer and afourth layer, the first layer and the second layer between the thirdlayer and the fourth layer; and the plurality of vias comprise buriedvias. Additionally or alternatively, in some examples, the PCB comprisesa third layer and a fourth layer, the first layer and the second layerabove the third layer and the fourth layer or the first layer and thesecond layer below the third layer and the fourth layer; and theplurality of vias comprise blind vias. Additionally or alternatively, insome examples, the sensing circuitry comprises: a differential amplifiercoupled to the thermopile, wherein a first terminal of the thermopile iscoupled to a first input of the differential amplifier and a secondterminal of the thermopile is coupled to a second input of theamplifier, different from the first input of the amplifier. Additionallyor alternatively, in some examples, the sensing circuitry furthercomprises: an analog-to-digital converter coupled to an output of thedifferential amplifier configured to convert the output to a digitalsignal. Additionally or alternatively, in some examples, the sensingcircuitry is mounted on a surface of the PCB. Additionally oralternatively, in some examples, the first Seebeck coefficient is apositive Seebeck coefficient and the second Seebeck coefficient is anegative Seebeck coefficient. Additionally or alternatively, in someexamples, the first conductive material is copper and the secondconductive material is constantan. Additionally or alternatively, insome examples, the plurality of conductive traces comprises copper.Additionally or alternatively, in some examples, the PCB has a thicknessgreater than 300 micron. Additionally or alternatively, in someexamples, the PCB has a thickness greater than 1 millimeter.

Some examples are directed to an electronic device including an absolutetemperature sensor configured to estimate a first temperature; a heatflux sensor comprising: one or more printed circuit boards (PCBs)comprising a thermopile, wherein the thermopile comprises a plurality ofthermocouples in series, and wherein the one or more PCBs are rigid; aplurality of vias in the one or more PCBs comprising one or more firstvias from a first layer of the one or more PCBs to a second layer of theone or more PCBs comprising a first conductive material with a firstSeebeck coefficient and one or more second vias from the first layer ofthe one or more PCBs to the second layer of the one or more PCBscomprising a second conductive material with a second Seebeckcoefficient, different from the first Seebeck coefficient; a pluralityof conductive traces on the first layer of the one or more PCB s and onthe second layer of the one or more PCBs, the plurality of conductivetraces interconnecting the plurality of vias; and sensing circuitrycoupled to the thermopile and configured to measure a signalproportional to a temperature gradient, such as an electrical propertysuch as a voltage signal, or another type of signal, between the firstlayer of the one or more PCBs and the second layer of the one or morePCBs; and processing circuitry coupled to the sensing circuitry and theabsolute temperature sensor, the processing circuitry configured toestimate a second temperature using the first temperature and the signalproportional to the temperature gradient. Additionally or alternatively,in some examples, the absolute temperature sensor comprises: a negativetemperature coefficient (NTC) temperature sensor, a resistancetemperature detector (RTD), or a diode based temperature sensor.Additionally or alternatively, in some examples, the electronic devicefurther includes a housing; wherein the absolute temperature sensor isdisposed within the housing and configured to estimate the firsttemperature at a first location within the housing. Additionally oralternatively, in some examples, the electronic device is a wearabledevice, and the second temperature is a skin temperature at a point ofcontact between a user's skin and the wearable device. Additionally oralternatively, in some examples, the second temperature is an ambienttemperature external to the electronic device. Additionally oralternatively, in some examples, the first temperature corresponds to afirst location within the electronic device and the second temperaturecorresponds to a second location with the electronic device, the secondlocated different than the first location. Additionally oralternatively, in some examples, the second location is separated fromthe first location by the one or more PCBs. Additionally oralternatively, in some examples, the electronic device further includesa display comprising the one or more PCBs. Additionally oralternatively, in some examples, the electronic device further includesan optical sensor comprising: one or more photo emitters; and one ormore photodetectors; and wherein the one or more PCBs of heat fluxsensor comprise a first PCB and a second PCB, wherein the one or morephoto emitters and the one or more photodetectors of the optical sensorare disposed on the first PCB and the second PCB comprises an opticalspacer between the optical sensor and a back crystal of the electronicdevice and wherein the second temperature corresponds to the opticalsensor. Additionally or alternatively, in some examples, the first PCBand the second PCB are coupled together with an adhesive. Additionallyor alternatively, in some examples, the processing circuitry is furtherconfigured to estimate a wavelength of at least one of the one or morephoto emitters. Additionally or alternatively, in some examples, theprocessing circuitry is further configured to estimate a physiologicalcharacteristic using the optical sensor and the wavelength of the atleast one of the one or more photo emitters. Additionally oralternatively, in some examples, the one or more PCBs have a thicknessgreater than 300 micron. Additionally or alternatively, in someexamples, the one or more PCBs have a thickness greater than 1millimeter.

Some examples are directed to a method comprising: measuring a firsttemperature at a first location using an absolute temperature sensor;estimating a heat flux through a rigid printed circuit board comprisinga plurality of vias comprising one or more first vias comprising a firstconductive material with a first Seebeck coefficient and one or moresecond vias comprising a second conductive material with a secondSeebeck coefficient, different from the first Seebeck coefficient,wherein the plurality of vias are interconnected to form a thermopile;and estimating a second temperature at a second location different fromthe first location. Additionally or alternatively, in some examples,estimating the heat flux through the rigid printed circuit boardcomprises: measuring a differential voltage across the thermopile; andestimating the heat flux using the differential voltage, a thermalresistance of the rigid printed circuit board, and a thermoelectricsensitivity of the thermopile. Additionally or alternatively, in someexamples, the second temperature measures a skin temperature at a pointof contact between a user's skin and an electronic device comprising therigid printed circuit board. Additionally or alternatively, in someexamples, the second temperature is an ambient temperature external toan electronic device comprising the rigid printed circuit board.Additionally or alternatively, in some examples, the first temperaturecorresponds to a first location within an electronic device comprisingthe rigid printed circuit board and the second temperature correspondsto a second location with the electronic device, the second locatedseparated from the first location by the rigid printed circuit board.

Some examples of the disclosure are directed to a heat flux sensorincluding a flexible printed circuit board (PCB) including a thermopile,wherein the thermopile comprises a plurality of thermocouples in series,the flexible PCB comprising: a first conductive layer comprising a firstconductive material with a first Seebeck coefficient; a secondconductive layer comprising a second conductive material with a secondSeebeck coefficient, different from the first Seebeck coefficient; and aplurality of vias between the first conductive layer and the secondconductive layer; and sensing circuitry coupled to the thermopile andconfigured to measure a voltage proportional to a temperature gradientbetween a first end of the flexible PCB and a second end of flexiblePCB. Additionally or alternatively, in some examples, the firstconductive layer is patterned with a first plurality of conductivetraces of the first conductive material and the second conductive layeris patterned with a second plurality of conductive traces of the secondconductive material. Additionally or alternatively, in some examples,each of the plurality of thermocouples comprises one of the firstplurality of conductive traces and one of the second plurality ofconductive traces coupled by one of the plurality of vias. Additionallyor alternatively, in some examples, the plurality of vias includes aplurality of first vias and a plurality of second vias, wherein theplurality of first vias are disposed within a first threshold distanceof the first end of the flexible PCB and wherein the plurality of secondvias are disposed within a second threshold distance of the second endof the flexible PCB. Additionally or alternatively, in some examples,the first threshold distance and the second threshold distance are lessthan 50 micron. Additionally or alternatively, in some examples, a firstconductive trace of the first plurality of conductive traces and secondconductive trace of the first plurality of conductive traces eachpartially overlap with a first conductive trace of the second pluralityof conductive traces, such that a first via of the plurality of viasthrough the flexible PCB electrically couples the first conductive traceof the first plurality of conductive traces to the first conductivetrace of the second plurality of conductive traces and a second via ofthe plurality of vias through the flexible PCB electrically couples thesecond conductive trace of the first plurality of conductive traces tothe first conductive trace of the second plurality of conductive traces.Additionally or alternatively, in some examples, the sensing circuitrycomprises: a differential amplifier coupled to the thermopile, wherein afirst terminal of the thermopile is coupled to a first input of thedifferential amplifier and a second terminal of the thermopile iscoupled to a second input of the amplifier, different from the firstinput of the amplifier. Additionally or alternatively, in some examples,the sensing circuitry further comprises: an analog-to-digital convertercoupled to an output of the differential amplifier configured to convertthe output to a digital signal. Additionally or alternatively, in someexamples, the sensing circuitry further comprises: a bias amplifierconfigured to generate a bias voltage as an output of the biasamplifier, wherein the output of the bias amplifier is coupled to afirst input of the differential amplifier and coupled as a referencevoltage to the analog-to-digital converter. Additionally oralternatively, in some examples, the sensing circuitry is mounted on asurface of a rigid PCB and the first end of the flexible PCB is coupledto the rigid PCB. Additionally or alternatively, in some examples, thefirst end of the flexible PCB is bonded to the rigid PCB with aconductive adhesive. Additionally or alternatively, in some examples,the first Seebeck coefficient is a positive Seebeck coefficient and thesecond Seebeck coefficient is a negative Seebeck coefficient.Additionally or alternatively, in some examples, the first conductivematerial is copper and the second conductive material is constantan.Additionally or alternatively, in some examples, the flexible PCBincludes one or more signal traces or one or more power tracesindependent of the thermopile, the one or more signal traces or the oneor more power traces configured to route one or more signals or one ormore power sources from the first end of the flexible PCB to the secondend of the flexible PCB. Additionally or alternatively, in someexamples, the flexible circuit comprises a third conductive layercomprising the first conductive material, the first conductive layerbetween the second conductive layer and the third conductive layer; theone or more signal traces are implemented in the first conductive layerusing the first conductive material and are shielded by the secondconductive material in the second conductive layer and by the firstconductive material in the third conductive layer. Additionally oralternatively, in some examples, the flexible circuit comprises a thirdconductive layer comprising the first conductive material, the secondconductive layer between the first conductive layer and the thirdconductive layer; the one or more signal traces are implemented in thesecond conductive layer using the second conductive material and areshielded by the first conductive material in the first conductive layerand by the first conductive material in the third conductive layer.

Some examples are directed to an electronic device comprising: a rigidprinted circuit board (PCB); an absolute temperature sensor configuredto estimate a first temperature and coupled to the rigid PCB; a heatflux sensor comprising: a flexible PCB including a thermopile, whereinthe thermopile comprises a plurality of thermocouples in series, theflexible PCB comprising: a first conductive layer comprising a firstconductive material with a first Seebeck coefficient; a secondconductive layer comprising a second conductive material with a secondSeebeck coefficient, different from the first Seebeck coefficient; and aplurality of vias between the first conductive layer and the secondconductive layer; and sensing circuitry coupled to the thermopileconfigured to measure a voltage proportional to a temperature gradientbetween a first end of the flexible PCB coupled to the rigid PCB and asecond end of flexible PCB; and processing circuitry coupled to thesensing circuitry and the absolute temperature sensor, the processingcircuitry configured to estimate a second temperature using the firsttemperature and the signal proportional to the temperature gradient.Additionally or alternatively, in some examples, the absolutetemperature sensor comprises: a negative temperature coefficient (NTC)temperature sensor, a resistance temperature detector (RTD), or a diodebased temperature sensor. Additionally or alternatively, in someexamples, the electronic device further includes a housing; wherein theabsolute temperature sensor is disposed within the housing andconfigured to estimate the first temperature at a first location withinthe housing. Additionally or alternatively, in some examples, theelectronic device is a wearable device, and the second temperature is askin temperature at a point of contact between a user's skin and thewearable device. Additionally or alternatively, in some examples, thesecond temperature is an ambient temperature external to the electronicdevice. Additionally or alternatively, in some examples, the firsttemperature corresponds to a first location within the electronic deviceand the second temperature corresponds to a second location with theelectronic device, the second located different than the first location.Additionally or alternatively, in some examples, the second location isa back crystal of the electronic device. Additionally or alternatively,in some examples, the second location is a second rigid PCB of theelectronic device. Additionally or alternatively, in some examples, theelectronic device further includes an optical sensor comprising: one ormore photo emitters; and one or more photodetectors; and wherein thefirst end of the flexible PCB or the second end of the flexible PCB iscoupled to the optical sensor. Additionally or alternatively, in someexamples, the one or more photo emitters and the one or more photodetectors are coupled to the rigid PCB. Additionally or alternatively,in some examples, the one or more photo emitters and the one or morephotodetectors are coupled to a second PCB of the electronic device, thefirst end of the flexible PCB is coupled to the second rigid PCB, andthe second end of the flexible PCB is coupled to the rigid PCB.

Some examples are directed to a method comprising: measuring a firsttemperature at a first location using an absolute temperature sensor;estimating a heat flux across a flexible printed circuit boardcomprising a first conductive layer having a first conductive materialwith a first Seebeck coefficient, a second conductive layer having asecond conductive material with a second Seebeck coefficient, differentfrom the first Seebeck coefficient, and a plurality of viasinterconnecting segments of the first conductive material and segmentsof the second conductive material to form a thermopile; and estimating asecond temperature at a second location different from the firstlocation. Additionally or alternatively, in some examples, estimatingthe heat flux across the flexible printed circuit board comprises:measuring a differential voltage across the thermopile; and estimatingthe heat flux using the differential voltage, a thermal resistance ofthe flexible printed circuit board, and a thermoelectric sensitivity ofthe thermopile. Additionally or alternatively, in some examples, thesecond temperature measures a skin temperature at a point of contactbetween a user's skin and an electronic device comprising the flexibleprinted circuit board. Additionally or alternatively, in some examples,the second temperature is an ambient temperature external to anelectronic device comprising the flexible printed circuit board.

Some examples of the disclosure are directed to measuring a firsttemperature at a first location using an absolute temperature sensor;estimating a temperature differential across a flexible printed circuitboard (PCB) comprising a thermopile; estimating a second temperature ata second location, different from the first location, corresponding to afirst light emitter; and estimating a wavelength of light emitted fromthe first light emitter based on the second temperature. Additionally oralternatively, in some examples the method further includes adjusting adriving parameter of the first light emitter based on the estimatedwavelength of light emitted from the first light emitter. Additionallyor alternatively, in some examples adjusting the driving parametercomprises adjusting a drive current applied to the first light emitter.Additionally or alternatively, in some examples the method includescompensating an estimation of a physiological characteristic from lightdetected by a photodetector based on the estimated wavelength of lightemitted from the first light emitter. Additionally or alternatively, insome examples the method includes estimating a second temperaturedifferential across a second flexible printed circuit board (PCB)comprising a second thermopile; estimating a third temperature at athird location, different from the first location and the secondlocation, corresponding to a second light emitter; and estimating awavelength of light emitted from the second light emitter based on thesecond temperature. Additionally or alternatively, in some examples themethod includes adjusting a driving parameter of the first light emitterbased on the estimated wavelength of light emitted from the first lightemitter and/or a driving parameter of the second light emitter based onthe estimated wavelength of light emitted from the second light emitter;and/or compensating an estimation of a physiological characteristic fromlight detected by one or more photodetectors based on the estimatedwavelength of light emitted from the first light emitter and/or based onthe estimated wavelength of light emitted from the second light emitter.Additionally or alternatively, in some examples the flexible PCB furthercomprises a second thermopile, the method further comprising: estimatinga second temperature differential across the flexible PCB using thesecond thermopile; estimating a third temperature at a third location,different from the first location and the second location, correspondingto a second light emitter; and estimating a wavelength of light emittedfrom the second light emitter based on the second temperature.Additionally or alternatively, in some examples the method includesadjusting a driving parameter of the first light emitter based on theestimated wavelength of light emitted from the first light emitterand/or a driving parameter of the second light emitter based on theestimated wavelength of light emitted from the second light emitter;and/or compensating an estimation of a physiological characteristic fromlight detected by one or more photodetectors based on the estimatedwavelength of light emitted from the first light emitter and/or based onthe estimated wavelength of light emitted from the second light emitter.Additionally or alternatively, in some examples the flexible PCBcomprises a first conductive layer having a first conductive materialwith a first Seebeck coefficient, a second conductive layer having asecond conductive material with a second Seebeck coefficient, differentfrom the first Seebeck coefficient, and a plurality of viasinterconnecting segments of the first conductive material and segmentsof the second conductive material to form the thermopile. It should benoted that the wavelength estimation techniques disclosed herein can beapplied to any LED/PD (e.g., light emitting diode/photodetector orphotodiode components that are located at any location in the device.

Some examples of the disclosure are directed to a first photo emitter; afirst photodetector; a first flexible printed circuit board (PCB)including a first thermopile; optical circuitry configured to drive thefirst photo emitter and measure the first photodetector; an absolutetemperature sensor configured to estimate a first temperature;thermopile sensing circuitry coupled to the first thermopile andconfigured to measure a signal proportional to a temperature gradient,such as an electrical property such as a voltage signal, or another typeof signal, between a first end of the first flexible PCB and a secondend of first flexible PCB; and processing circuitry coupled to thethermopile sensing circuitry and the absolute temperature sensor, theprocessing circuitry configured to: estimate a second temperature at thefirst photo emitter using the first temperature and the signalproportional to the temperature gradient between the first end of thefirst flexible PCB and the second end of first flexible PCB; andestimate a wavelength of light emitted by the first photo emitter.Additionally or alternatively, in some examples the processing circuitryfurther configured to: adjust a driving parameter of the first lightemitter using the estimated wavelength of light emitted from the firstphoto emitter. Additionally or alternatively, in some examples adjustingthe driving parameter comprises adjusting a drive current applied to thefirst photo emitter. Additionally or alternatively, in some examples theprocessing circuitry is further configured to: compensate an estimationof a physiological characteristic from light detected by a photodetectorbased on the estimated wavelength of light emitted from the first photoemitter. Additionally or alternatively, in some examples the opticalcircuitry, the absolute temperature sensor, the thermopile sensingcircuitry and the processing circuitry are integrated in an integratedcircuit. Additionally or alternatively, in some examples the first photoemitter and the first photodetector are disposed on the first flexiblePCB at the second end of the first flexible PCB. Additionally oralternatively, in some examples the first flexible PCB comprises routingtrace configured to route the first photo emitter and the firstphotodetector to the optical circuitry. Additionally or alternatively,in some examples the electronic device further includes a second photoemitter; a second photodetector; a second flexible PCB including asecond thermopile; wherein: the optical circuitry is further configuredto drive the second photo emitter and measure the second photodetector;the thermopile sensing circuitry is coupled to the second thermopile andconfigured to measure a signal proportional to a temperature gradient,such as an electrical property such as a voltage signal, or another typeof signal, between a first end of the second flexible PCB and a secondend of the second flexible PCB; and the processing circuitry is furtherconfigured to: estimate a third temperature at the second photo emitterusing the first temperature and the signal proportional to thetemperature gradient between the first end of the second flexible PCBand the second end of the second flexible PCB; and estimate a wavelengthof light emitted by the second photo emitter. Additionally oralternatively, in some examples the second photo emitter and the secondphotodetector are disposed on the second flexible PCB at the second endof the second flexible PCB. Additionally or alternatively, in someexamples the electronic device further includes a second photo emitter;a second photodetector; wherein: the first flexible PCB furtherincluding a second thermopile; the optical circuitry is furtherconfigured to drive the second photo emitter and measure the secondphotodetector; the thermopile sensing circuitry is coupled to the secondthermopile and configured to measure a signal proportional to atemperature gradient, such as an electrical property such as a voltagesignal, or another type of signal, between a first end of the firstflexible PCB and a second end of first flexible PCB; and the processingcircuitry is further configured to: estimate a third temperature at thesecond photo emitter using the first temperature and the signalproportional to the temperature gradient between the first end of thefirst flexible PCB and the second end of the first flexible PCB; andestimate a wavelength of light emitted by the second photo emitter.Additionally or alternatively, in some examples the first thermopilecomprises a plurality of thermocouples in series, the first flexible PCBcomprising: a first conductive layer comprising a first conductivematerial with a first Seebeck coefficient; a second conductive layercomprising a second conductive material with a second Seebeckcoefficient, different from the first Seebeck coefficient; and aplurality of vias between the first conductive layer and the secondconductive layer. Additionally or alternatively, in some examples theelectronic device is a wearable device.

Some examples are directed to a method comprising: monitoring one ormore first sensors; in accordance with a determination that one or morecriteria are satisfied, estimating a body temperature of a user, whereinestimating the body temperature of the user comprises: measuring a firsttemperature at a first location with an electronic device using anabsolute temperature sensor; estimating a temperature differentialacross a flexible printed circuit board (PCB) comprising a thermopile;and estimating a second temperature at a second location, different fromthe first location; and in accordance with a determination that the oneor more criteria are not satisfied, forgoing estimating the bodytemperature of a user. Additionally or alternatively, in some examplesthe one or more first sensors comprise a touch sensor and the one ormore first criteria includes a criterion that is satisfied when anobject touches the touch sensor. Additionally or alternatively, in someexamples the one or more first sensors comprise a touch sensor and theone or more first criteria includes a criterion that is satisfied whenan object touches greater than threshold area of the touch sensor.Additionally or alternatively, in some examples the one or more firstsensors comprise a motion and/or orientation sensor. Additionally oralternatively, in some examples the motion and/or orientation sensorcomprises an inertial measurement unit, an accelerometer, or agyroscope. Additionally or alternatively, in some examples the one ormore first criteria include a criterion that is satisfied when themotion and/or orientation sensor indicates less that a threshold amountof motion. Additionally or alternatively, in some examples the one ormore first criteria include a criterion that is satisfied when themotion and/or orientation sensor indicates a motion of a wrist of theuser to a forehead of the user. Additionally or alternatively, in someexamples the one or more first sensors comprise the absolute temperaturesensor and/or the thermopile, and wherein the one or more first criteriainclude an absolute temperature above a first temperature thresholdand/or a temperature differential above a second temperature threshold.

Some examples are directed to an electronic device comprising: anabsolute temperature sensor configured to estimate a first temperatureat a first location within the electronic device; a flexible PCBincluding a thermopile; sensing circuitry coupled to the thermopile andconfigured to measure a voltage proportional to a temperature gradientacross the thermopile; and processing circuitry coupled to the sensingcircuitry and the absolute temperature sensor, the processing circuitryconfigured to: monitoring one or more first sensors; in accordance witha determination that one or more criteria are satisfied, estimating abody temperature of a user, wherein estimating the body temperature ofthe user comprises: measuring the first temperature at the firstlocation using the absolute temperature sensor; estimating a temperaturedifferential across the thermopile; and estimating a second temperatureat a second location, different from the first location; and inaccordance with a determination that the one or more criteria are notsatisfied, forgoing estimating the body temperature of a user.

Some examples of the disclosure are directed to a method comprising:monitoring one or more first sensors; in accordance with a determinationthat one or more criteria are satisfied, estimating a body temperatureof a user, wherein estimating the body temperature of the usercomprises: measuring a first temperature at a first location with anelectronic device using an absolute temperature sensor; estimating atemperature differential across a flexible printed circuit board (PCB)comprising a thermopile; and estimating a second temperature at a secondlocation, different from the first location; and in accordance with adetermination that the one or more criteria are not satisfied, forgoingestimating the body temperature of a user.

Additionally or alternatively, in some examples the one or more firstsensors comprise a touch sensor and the one or more criteria includes acriterion that is satisfied when an object touches the touch sensor.Additionally or alternatively, in some examples the one or more firstsensors comprise a touch sensor and the one or more criteria includes acriterion that is satisfied when an object contacts greater than athreshold area of the touch sensor. Additionally or alternatively, insome examples the one or more first sensors comprise a motion and/ororientation sensor. Additionally or alternatively, in some examples themotion and/or orientation sensor comprises an inertial measurement unit,an accelerometer, or a gyroscope. Additionally or alternatively, in someexamples the one or more criteria include a criterion that is satisfiedwhen the motion and/or orientation sensor indicates less than athreshold amount of motion. Additionally or alternatively, in someexamples the one or more criteria include a criterion that is satisfiedwhen the motion and/or orientation sensor indicates a motion of a wristof the user to a forehead of the user. Additionally or alternatively, insome examples the one or more first sensors comprise the absolutetemperature sensor and/or the thermopile, and wherein the one or morecriteria include an absolute temperature above a first temperaturethreshold and/or a temperature differential above a second temperaturethreshold.

Some examples of the disclosure are directed toward an electronic devicecomprising: an absolute temperature sensor configured to estimate afirst temperature at a first location within the electronic device; aflexible PCB including a thermopile; sensing circuitry coupled to thethermopile and configured to measure a voltage proportional to atemperature gradient across the thermopile; and processing circuitrycoupled to the sensing circuitry and the absolute temperature sensor,the processing circuitry configured for: monitoring one or more firstsensors; in accordance with a determination that one or more criteriaare satisfied, estimating a body temperature of a user, whereinestimating the body temperature of the user comprises: measuring thefirst temperature at the first location using the absolute temperaturesensor; estimating a temperature differential across the thermopile; andestimating a second temperature at a second location, different from thefirst location; and in accordance with a determination that the one ormore criteria are not satisfied, forgoing estimating the bodytemperature of a user.

Some examples of the disclosure are directed toward a method comprising:receiving, at a device, a request to initiate a temperature measurement;in accordance with a determination that one or more criteria aresatisfied, initiating the temperature measurement; and in accordancewith a determination that the one or more criteria are not satisfied:implementing, at the device, one or more measures to satisfy the one ormore criteria, and after implementing the one or measures, initiatingthe temperature measurement.

Additionally or alternatively, in some examples the one or more criteriainclude a criterion based on operating conditions associated with thedevice, the method further comprising: in accordance with thedetermination that the one or more criteria are satisfied: monitoringthe operating conditions associated with the device, in response toinitiating the temperature measurement; and compensating the temperaturemeasurement based on the operating conditions. Additionally oralternatively, in some examples compensating the temperature measurementbased on the operating conditions comprises: generating an adjustedtemperature measurement by applying a temperature compensation modelbased on the operating conditions, to the temperature measurement.Additionally or alternatively, in some examples the temperaturecompensation model is based on a power draw associated with the deviceconditions, the method further comprising: displaying the adjustedtemperature measurement. Additionally or alternatively, in some examplesimplementing the one or more measures comprises: instructing a user toadjust an orientation of the device relative to the user's skin.Additionally or alternatively, in some examples implementing the one ormore measures comprises: instructing a user to reduce motion of thedevice. Additionally or alternatively, in some examples implementing theone or more measures comprises: instructing a user to modifyinteractions with the device, to minimize usage of device componentsassociated with heat dissipation. Additionally or alternatively, in someexamples implementing the one or more measures comprises: suppressing,at the device, power delivery to device components associated with heatdissipation. Additionally or alternatively, in some examples the devicecomponents associated with heat dissipation comprise a processor, adisplay, an antenna, cellular communication circuitry, Wi-Fi circuitry,Bluetooth circuitry, or GPS circuitry. Additionally or alternatively, insome examples the method further comprises: completing the temperaturemeasurement; and restoring power delivery to device componentsassociated with heat dissipation, in response to completing thetemperature measurement. Additionally or alternatively, in some examplesimplementing the one or more measures comprises: changing an operationalmode of the device to a lower-power mode associated with lower heatdissipation.

Some examples are directed to an electronic device comprising any of theheat flux sensors described above.

Some examples of the disclosure are directed to a non-transitorycomputer readable storage medium storing instructions, which whenexecuted by an electronic device including processing circuitry, causethe device to perform any of the methods described above.

The present disclosure contemplates that the entities responsible forthe collection, analysis, disclosure, transfer, storage, or other use ofsuch personal data will comply with well-established privacy policiesand/or privacy practices. In particular, such entities should implementand consistently use privacy policies and practices that are generallyrecognized as meeting or exceeding industry or governmental requirementsfor maintaining personal information data private and secure. Suchpolicies should be easily accessible by users, and should be updated asthe collection and/or use of data changes. Personal information fromusers should be collected for legitimate and reasonable uses of theentity and not shared or sold outside of those legitimate uses. Further,such collection/sharing should require receipt of the informed consentof the users. Additionally, such entities should consider taking anyneeded steps for safeguarding and securing access to such personalinformation data and ensuring that others with access to the personalinformation data adhere to their privacy policies and procedures.Further, such entities can subject themselves to evaluation by thirdparties to certify their adherence to widely accepted privacy policiesand practices. The policies and practices may be adapted depending onthe geographic region and/or the particular type and nature of personaldata being collected and used.

Despite the foregoing, the present disclosure also contemplates examplesin which users selectively block the collection of, use of, or accessto, personal data, including physiological information. For example, auser may be able to disable hardware and/or software elements thatcollect physiological information. Further, the present disclosurecontemplates that hardware and/or software elements can be provided toprevent or block access to personal data that has already beencollected. Specifically, users can select to remove, disable, orrestrict access to certain health-related applications collecting users'personal health or fitness data.

Although the disclosed examples have been fully described with referenceto the accompanying drawings, it is to be noted that various changes andmodifications will become apparent to those skilled in the art. Suchchanges and modifications are to be understood as being included withinthe scope of the disclosed examples as defined by the appended claims.

1. A heat flux sensor comprising: a printed circuit board (PCB)comprising a thermopile, wherein the thermopile comprises a plurality ofthermocouples in series, and wherein the PCB is rigid; a plurality ofvias in the PCB comprising one or more first vias from a first layer ofthe PCB to a second layer of the PCB comprising a first conductivematerial with a first Seebeck coefficient and one or more second viasfrom the first layer of the PCB to the second layer of the PCBcomprising a second conductive material with a second Seebeckcoefficient, different from the first Seebeck coefficient; a pluralityof conductive traces on the first layer of the PCB and on the secondlayer of the PCB, the plurality of conductive traces interconnecting theplurality of vias; and sensing circuitry coupled to the thermopile andconfigured to measure a voltage proportional to a temperature gradientbetween the first layer and the second layer of the PCB.
 2. The heatflux sensor of claim 1, wherein each of the plurality of thermocouplescomprises one of the first vias and one of the second vias coupled byone of the plurality of conductive traces.
 3. The heat flux sensor ofclaim 1, wherein the plurality of vias comprise through-hole vias fromthe first layer to the second layer.
 4. The heat flux sensor of claim 1,wherein: the PCB comprises a third layer and a fourth layer, the thirdlayer and the fourth layer between the first layer and the second layer;and the plurality of vias comprise through-hole vias.
 5. The heat fluxsensor of claim 1, wherein: the PCB comprises a third layer and a fourthlayer, the first layer and the second layer between the third layer andthe fourth layer; and the plurality of vias comprise buried vias.
 6. Theheat flux sensor of claim 1, wherein: the PCB comprises a third layerand a fourth layer, the first layer and the second layer above the thirdlayer and the fourth layer or the first layer and the second layer belowthe third layer and the fourth layer; and the plurality of vias compriseblind vias.
 7. The heat flux sensor of claim 1, wherein the sensingcircuitry is mounted on a surface of the PCB.
 8. The heat flux sensor ofclaim 1, wherein the first Seebeck coefficient is a positive Seebeckcoefficient and the second Seebeck coefficient is a negative Seebeckcoefficient.
 9. The heat flux sensor of claim 1, wherein the firstconductive material is copper and the second conductive material isconstantan.
 10. The heat flux sensor of claim 1, wherein the PCB has athickness greater than 300 micron.
 11. The heat flux sensor of claim 1,wherein the PCB has a thickness greater than 1 millimeter.
 12. Anelectronic device comprising: an absolute temperature sensor configuredto estimate a first temperature; a heat flux sensor comprising: one ormore printed circuit boards (PCBs) comprising a thermopile, wherein thethermopile comprises a plurality of thermocouples in series, and whereinthe one or more PCBs are rigid; a plurality of vias in the one or morePCB s comprising one or more first vias from a first layer of the one ormore PCBs to a second layer of the one or more PCBs comprising a firstconductive material with a first Seebeck coefficient and one or moresecond vias from the first layer of the one or more PCBs to the secondlayer of the one or more PCBs comprising a second conductive materialwith a second Seebeck coefficient, different from the first Seebeckcoefficient; a plurality of conductive traces on the first layer of theone or more PCBs and on the second layer of the one or more PCBs, theplurality of conductive traces interconnecting the plurality of vias;and sensing circuitry coupled to the thermopile and configured tomeasure a signal proportional to a temperature gradient between thefirst layer of the one or more PCBs and the second layer of the one ormore PCBs; and processing circuitry coupled to the sensing circuitry andthe absolute temperature sensor, the processing circuitry configured toestimate a second temperature using the first temperature and the signalproportional to the temperature gradient.
 13. The electronic device ofclaim 12, wherein the second temperature is an ambient temperatureexternal to the electronic device.
 14. The electronic device of claim12, wherein the first temperature corresponds to a first location withinthe electronic device and the second temperature corresponds to a secondlocation with the electronic device, the second location different thanthe first location.
 15. The electronic device of claim 14, wherein thesecond location is separated from the first location by the one or morePCBs.
 16. The electronic device of claim 12, further comprising: adisplay comprising the one or more PCBs.
 17. The electronic device ofclaim 12, further comprising: an optical sensor comprising: one or morephoto emitters; and one or more photodetectors; wherein: the one or morePCBs of the heat flux sensor comprise a first PCB and a second PCB, theone or more photo emitters and the one or more photodetectors of theoptical sensor are disposed on the first PCB, the second PCB comprisesan optical spacer between the optical sensor and a back crystal of theelectronic device, and the second temperature corresponds to the opticalsensor.
 18. The electronic device of claim 17, wherein the processingcircuitry is further configured to estimate a physiologicalcharacteristic using the optical sensor and a wavelength of at least oneof the one or more photo emitters.
 19. A method comprising: measuring afirst temperature at a first location using an absolute temperaturesensor; estimating a heat flux through a rigid printed circuit boardcomprising a plurality of vias comprising one or more first viascomprising a first conductive material with a first Seebeck coefficientand one or more second vias comprising a second conductive material witha second Seebeck coefficient, different from the first Seebeckcoefficient, wherein the plurality of vias are interconnected to form athermopile; and estimating a second temperature at a second locationdifferent from the first location.
 20. The method of claim 19, whereinestimating the heat flux through the rigid printed circuit boardcomprises: measuring a differential voltage across the thermopile; andestimating the heat flux using the differential voltage, a thermalresistance of the rigid printed circuit board, and a thermoelectricsensitivity of the thermopile.
 21. The method of claim 19, wherein thesecond temperature measures a skin temperature at a point of contactbetween a user's skin and an electronic device comprising the rigidprinted circuit board.
 22. The method of claim 19, wherein the secondtemperature is an ambient temperature external to an electronic devicecomprising the rigid printed circuit board.
 23. The method of claim 19,wherein the first temperature corresponds to a first location within anelectronic device comprising the rigid printed circuit board and thesecond temperature corresponds to a second location with the electronicdevice, the second location separated from the first location by therigid printed circuit board.